[Gecko Bug 1775501] multipart boundary should be handled as mixed case

[moz-wptsync-bot]

Since w3c/FileAPI#43 is still open, it is unclear how body.type should work.
The current wpts expect some behavior which isn't in the specifications.
So, since the situation is very messy in the specifications, the patch is doing a
spot fix for boundary handling. It is ugly, but shouldn't change other behavior.

Differential Revision: https://phabricator.services.mozilla.com/D150018

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1775501
gecko-commit: 22a157de1bfa502e10ec05c9b17611f2eba47b0e
gecko-reviewers: kershaw

[wpt-pr-bot]

The review process for this patch is being conducted in the Firefox project.

[jimmywarting]

  const r = new Response(new Blob([form_string]), {
    headers: [
      [
        "Content-Type",
        "multipart/form-data; boundary=Boundary_with_capital_letters",
      ],
    ],

I think there should also be a test case where it sets the blob type and the response set's the content-type out of the blob.type... like this:

const blob = new Blob([form_string], { type: "multipart/form-data; boundary=Boundary_with_capital_letters" })
new Response(blob)