Cannot specify preview API accept header with non-json type

[haya14busa]

For example, GitHub apps API need preview accept header application/vnd.github.machine-man-preview+json
https://developer.github.com/apps/building-github-apps/authenticating-with-github-apps/

If the GitHub apps need to access non-json type endpoint such as getting diff of PullRequest, need to set
application/vnd.github.machine-man-preview.diff
as documented in https://developer.github.com/v3/media/#diff. (application/vnd.github.VERSION.diff)

However, go-github always set application/vnd.github.v3.diff for PullRequests.GetRaw request, so the request to GitHub will fail with 403 Resource not accessible by integration error.

I confirmed it works if i use application/vnd.github.machine-man-preview.diff accept header.

Note that it doesn't work only for private repository and as for public repo, it works with application/vnd.github.v3.diff header.

[haya14busa]

Sorry, i thought i confirmed it works but it actually not. maybe GitHub problem...