Undici allows setting forbidden headers

[infinity0]

• https://developer.mozilla.org/en-US/docs/Glossary/Forbidden_header_name
• https://fetch.spec.whatwg.org/#forbidden-request-header

Results in bugs like #2318.

There seems to be some disagreement on whether server environments should follow the WhatWG spec in this area, e.g. denoland/deno#6868. But if you decide this is "not a bug and working as intended", then #2318 is a bug. If you decide this is a bug and fix it, then you can decide that #2318 is not a bug and close it.