master_branch is not always returned

According to the [documentation](developer.github.com/v3/#detailed-representations):

    The documentation provides an example response 
    for each API method. The example response 
    illustrates all attributes that are returned by 
    that method.

Turns out that "master_branch" *is not* an attribute that is always returned by the Repo methods. 

More specifically, if you create a brand new repo on GitHub and don't push anything, that repo shows up in feeds like `/user/subscriptions` and it does not have a `master_branch` field.

If you feel this is not a bug in the documentation, but in the API, then ignore this commit... indeed, in my opinion, the API should always return that field, but simply set it to null if there is no master branch