How to send a custom header?

If you send `Accept: application/vnd.github.mercy-preview+json` then you get access to some extra stuff e.g. repository topics. So far I couldn't find any way to do this but I'm still searching through the code/docs.

The issue does not name a file or test; start by locating the JavaScript request entry point and checking how headers are currently constructed. Confirm whether callers can provide a custom Accept header, then document or test the usage for GitHub API preview media types such as repository topics.

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