Send correctly formatted Authorization header to github when OATH token is present

I ran into issues connecting to Github recently and was getting a `Faraday::ConnectionFailed` with no response when trying to create a pull request.  After hours of debugging I tried getting a list of all pull requests and Github was giving back an HTML response.

I hunted this down to it's source and [Github seems to suggest a different format](https://developer.github.com/v3/oauth/#use-the-access-token-to-access-the-api) for the `Authorization` header that is currently being used, and after I updated it here everything started magically working.

It also notes that the query params for the access token aren't needed if the header is present, but I didn't want to muck with removing that as I wasn't sure there was another purpose for overrides here (based on some of the specs you have).