Replaced usage of `typed_insert` since it ended converting `UTF-8` to
lowercase.
Removed one of the test cases since it wasn't following spec since
[xhr/205](https://github.com/whatwg/xhr/pull/205).
Testing: Changes covered by wpt
Fixes: #20436
---------
Signed-off-by: Gae24 <96017547+Gae24@users.noreply.github.com>