Fix issue where milestones API did not accept "all".

See GitHub API reference
  https://developer.github.com/v3/issues/milestones/#parameters

It is already documented that it is supported in Milestones#list
  https://github.com/peter-murach/github/blob/c71f4fb8f162e0d6bc12bc8ef0aa190f953ad65e/lib/github_api/client/issues/milestones.rb#L23
Merge pull request #186 from Fryguy/fix_milestones_all

Fix issue where milestones API did not accept "all".