Use the application/vnd.github.v3 content-type

http://developer.github.com/v3/media/

> Neither of these specify a version, so you will always get the latest JSON representation of resources. If you’re building an application and care about the stability of the API, specify a version like so:

I'm not sure if v3 is the correct version to use, I have not tested every single endpoint. Someone with more knowledge of this API client will have to confirm, but it should definitely use the correct content type. For compatibility reasons.