Make Response.redirect() headers mutable

[petamoriken]

Fixes #1868

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• At least two implementers are interested (and none opposed):
  • WebKit: Response.redirect should return object whose headers are mutable, as well as Response.json #1868 (comment)
  • …
• Tests are written and can be reviewed and commented upon at:
  • Test that Response.redirect() and Response.json() headers are mutable web-platform-tests/wpt#61634
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno (not for CORS changes): It has already been implemented that way.
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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