Prevent adding Accept header when already one.

[Eclion]

This change prevents the Accept header application/vnd.github.v3+json from being added to the headers in case there is already an Accept header.

The mentioned header is necessary for most interactions with the GitHub API, but it also influence the behaviour of some resource call in an unexpected way.

Example:
When downloading a release with the go-github library, an Accept header application/octet-stream is added by the latter, then ghinstallation adds the Accept header application/vnd.github.v3+json.
The response to such request isn't the content of the release, as one could expect when using the function DownloadReleaseAsset but a json describing the release.

This PR prevents this behaviour from happening and tackle the issue #12

[wlynch]

Thanks!