Send accept header when getting token

[btoews]

Some OAuth providers (Eg. GitHub) send a different response format depending on the Accept header from the token request. The default for GitHub is form encoded, but it can also return JSON and XML. GitHub's API can be made to work as-is by setting setTokenFormat(OAuth2.TOKEN_FORMAT.FORM_URL_ENCODED), but it would be cool if things worked out of the box. Since this library expects JSON by default, it would be nice if it sent Accept: application/json.

Github API OAuth Docs

Thanks for writing this library! It will be very helpful.