Headers of requests are bleeding into other plugins

[Ninerian]

I found a bug where a header field was still set in the request for another plugin (forge). This resulted in an error while using it. The extended discussion can be found here: https://github.com/magit/forge/issues/153

The explanation of the cause: https://github.com/magit/forge/issues/153#issuecomment-504331411

There is also a repo to test the bug: https://github.com/Ninerian/forge-test