Change to allow null value in "milestoneDescription".

`Milestone.milestoneDescription` is nullable.

example:
```haskell
import Github.Issues.Milestones

main :: IO ()
main = milestones "rails" "rails" >>= print
```

output:
```
Left (JsonError "when expecting a String, encountered Null instead on the JSON: Array (fromList [Object fromList [(\"open_issues\",Number 7),(\"title\",String \"4.1.0\"),(\"id\",Number 516000),(\"due_on\",Null),(\"closed_issues\",Number 74),(\"state\",String \"open\"),(\"updated_at\",String \"2014-01-26T21:30:06Z\"),(\"creator\",Object fromList [(\"organizations_url\",String \"https://api.github.com/users/rafaelfranca/orgs\"),(\"subscriptions_url\",String \"https://api.github.com/users/rafaelfranca/subscriptions\"),(\"followers_url\",String \"https://api.github.com/users/rafaelfranca/followers\"),(\"gists_url\",String \"https://api.github.com/users/rafaelfranca/gists{/gist_id}\"),(\"gravatar_id\",String \"0525b332aafb83307b32d9747a93de03\"),(\"received_events_url\",String \"https://api.github.com/users/rafaelfranca/received_events\"),(\"login\",String \"rafaelfranca\"),(\"starred_url\",String \"https://api.github.com/users/rafaelfranca/starred{/owner}{/repo}\"),(\"id\",Number 47848),(\"avatar_url\",String \"https://gravatar.com/avatar/0525b332aafb83307b32d9747a93de03?d=https%3A%2F%2Fidenticons.github.com%2F2c7b84fa30f0cea96a90729afe2970a0.png&r=x\"),(\"following_url\",String \"https://api.github.com/users/rafaelfranca/following{/other_user}\"),(\"repos_url\",String \"https://api.github.com/users/rafaelfranca/repos\"),(\"html_url\",String \"https://github.com/rafaelfranca\"),(\"site_admin\",Bool False),(\"url\",String \"https://api.github.com/users/rafaelfranca\"),(\"events_url\",String \"https://api.github.com/users/rafaelfranca/events{/privacy}\"),(\"type\",String \"User\")]),(\"labels_url\",String \"https://api.github.com/repos/rails/rails/milestones/29/labels\"),(\"description\",String \"\"),(\"number\",Number 29),(\"url\",String \"https://api.github.com/repos/rails/rails/milestones/29\"),(\"created_at\",String \"2013-12-17T20:57:35Z\")],Object fromList [(\"open_issues\",Number 57),(\"title\",String \"4.2.0\"),(\"id\",Number 297919),(\"due_on\",Null),(\"closed_issues\",Number 3),(\"state\",String \"open\"),(\"updated_at\",String \"2014-01-26T18:01:53Z\"),(\"creator\",Object fromList [(\"organizations_url\",String \"https://api.github.com/users/rafaelfranca/orgs\"),(\"subscriptions_url\",String \"https://api.github.com/users/rafaelfranca/subscriptions\"),(\"followers_url\",String \"https://api.github.com/users/rafaelfranca/followers\"),(\"gists_url\",String \"https://api.github.com/users/rafaelfranca/gists{/gist_id}\"),(\"gravatar_id\",String \"0525b332aafb83307b32d9747a93de03\"),(\"received_events_url\",String \"https://api.github.com/users/rafaelfranca/received_events\"),(\"login\",String \"rafaelfranca\"),(\"starred_url\",String \"https://api.github.com/users/rafaelfranca/starred{/owner}{/repo}\"),(\"id\",Number 47848),(\"avatar_url\",String \"https://gravatar.com/avatar/0525b332aafb83307b32d9747a93de03?d=https%3A%2F%2Fidenticons.github.com%2F2c7b84fa30f0cea96a90729afe2970a0.png&r=x\"),(\"following_url\",String \"https://api.github.com/users/rafaelfranca/following{/other_user}\"),(\"repos_url\",String \"https://api.github.com/users/rafaelfranca/repos\"),(\"html_url\",String \"https://github.com/rafaelfranca\"),(\"site_admin\",Bool False),(\"url\",String \"https://api.github.com/users/rafaelfranca\"),(\"events_url\",String \"https://api.github.com/users/rafaelfranca/events{/privacy}\"),(\"type\",String \"User\")]),(\"labels_url\",String \"https://api.github.com/repos/rails/rails/milestones/24/labels\"),(\"description\",String \"\"),(\"number\",Number 24),(\"url\",String \"https://api.github.com/repos/rails/rails/milestones/24\"),(\"created_at\",String \"2013-03-27T19:29:42Z\")],Object fromList [(\"open_issues\",Number 0),(\"title\",String \"Next Version\"),(\"id\",Number 426945),(\"due_on\",Null),(\"closed_issues\",Number 0),(\"state\",String \"open\"),(\"updated_at\",String \"2013-12-17T20:59:00Z\"),(\"creator\",Object fromList [(\"organizations_url\",String \"https://api.github.com/users/rafaelfranca/orgs\"),(\"subscriptions_url\",String \"https://api.github.com/users/rafaelfranca/subscriptions\"),(\"followers_url\",String \"https://api.github.com/users/rafaelfranca/followers\"),(\"gists_url\",String \"https://api.github.com/users/rafaelfranca/gists{/gist_id}\"),(\"gravatar_id\",String \"0525b332aafb83307b32d9747a93de03\"),(\"received_events_url\",String \"https://api.github.com/users/rafaelfranca/received_events\"),(\"login\",String \"rafaelfranca\"),(\"starred_url\",String \"https://api.github.com/users/rafaelfranca/starred{/owner}{/repo}\"),(\"id\",Number 47848),(\"avatar_url\",String \"https://gravatar.com/avatar/0525b332aafb83307b32d9747a93de03?d=https%3A%2F%2Fidenticons.github.com%2F2c7b84fa30f0cea96a90729afe2970a0.png&r=x\"),(\"following_url\",String \"https://api.github.com/users/rafaelfranca/following{/other_user}\"),(\"repos_url\",String \"https://api.github.com/users/rafaelfranca/repos\"),(\"html_url\",String \"https://github.com/rafaelfranca\"),(\"site_admin\",Bool False),(\"url\",String \"https://api.github.com/users/rafaelfranca\"),(\"events_url\",String \"https://api.github.com/users/rafaelfranca/events{/privacy}\"),(\"type\",String \"User\")]),(\"labels_url\",String \"https://api.github.com/repos/rails/rails/milestones/27/labels\"),(\"description\",Null),(\"number\",Number 27),(\"url\",String \"https://api.github.com/repos/rails/rails/milestones/27\"),(\"created_at\",String \"2013-09-13T13:30:09Z\")],Object fromList [(\"open_issues\",Number 52),(\"title\",String \"4.0.3\"),(\"id\",Number 458710),(\"due_on\",Null),(\"closed_issues\",Number 35),(\"state\",String \"open\"),(\"updated_at\",String \"2014-01-27T12:00:31Z\"),(\"creator\",Object fromList [(\"organizations_url\",String \"https://api.github.com/users/rafaelfranca/orgs\"),(\"subscriptions_url\",String \"https://api.github.com/users/rafaelfranca/subscriptions\"),(\"followers_url\",String \"https://api.github.com/users/rafaelfranca/followers\"),(\"gists_url\",String \"https://api.github.com/users/rafaelfranca/gists{/gist_id}\"),(\"gravatar_id\",String \"0525b332aafb83307b32d9747a93de03\"),(\"received_events_url\",String \"https://api.github.com/users/rafaelfranca/received_events\"),(\"login\",String \"rafaelfranca\"),(\"starred_url\",String \"https://api.github.com/users/rafaelfranca/starred{/owner}{/repo}\"),(\"id\",Number 47848),(\"avatar_url\",String \"https://gravatar.com/avatar/0525b332aafb83307b32d9747a93de03?d=https%3A%2F%2Fidenticons.github.com%2F2c7b84fa30f0cea96a90729afe2970a0.png&r=x\"),(\"following_url\",String \"https://api.github.com/users/rafaelfranca/following{/other_user}\"),(\"repos_url\",String \"https://api.github.com/users/rafaelfranca/repos\"),(\"html_url\",String \"https://github.com/rafaelfranca\"),(\"site_admin\",Bool False),(\"url\",String \"https://api.github.com/users/rafaelfranca\"),(\"events_url\",String \"https://api.github.com/users/rafaelfranca/events{/privacy}\"),(\"type\",String \"User\")]),(\"labels_url\",String \"https://api.github.com/repos/rails/rails/milestones/28/labels\"),(\"description\",String \"\"),(\"number\",Number 28),(\"url\",String \"https://api.github.com/repos/rails/rails/milestones/28\"),(\"created_at\",String \"2013-10-18T18:07:31Z\")]])")
```