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)
  • Gecko: Make Response.redirect() headers mutable #1944 (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: https://issues.chromium.org/issues/552469833
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2066547
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=322510
  • Deno (not for CORS changes): Make Response.redirect() headers mutable denoland/deno#36692
• MDN issue is filed: Document that Response.json() and Response.redirect() headers are mutable mdn/mdn#858
• The top of this comment includes a clear commit message to use.

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

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Preview | Diff

[annevk]

@valenting @ricea thoughts?

[valenting]

I am supportive of this change.

[annevk]

@petamoriken would you be willing to file browser bugs and complete OP? With WebKit & Gecko supportive I think we can land this.

[petamoriken]

@annevk DONE! 😄