multipart boundary should be handled as mixed case

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

fetch/content-type/multipart.window.js

@@ -0,0 +1,33 @@
+// META: title=Ensure capital letters can be used in the boundary value.
+setup({ single_test: true });
+(async () => {
+  const form_string =
+    "--Boundary_with_capital_letters\r\n" +
+    "Content-Type: application/json\r\n" +
+    'Content-Disposition: form-data; name="does_this_work"\r\n' +
+    "\r\n" +
+    'YES\r\n' +
+    "--Boundary_with_capital_letters--\r\n";
+
+  const r = new Response(new Blob([form_string]), {
+    headers: [
+      [
+        "Content-Type",
+        "multipart/form-data; boundary=Boundary_with_capital_letters",
+      ],
+    ],
+  });
+
+  var s = "";
+  try {
+    const fd = await r.formData();
+    for (const [key, value] of fd.entries()) {
+      s += (`${key} = ${value}`);
+    }
+  } catch (ex) {
+    s = ex;
+  }
+
+  assert_equals(s, "does_this_work = YES");
+  done();
+})();