Revert "[XHR] Do not replace request content-type "utf-8" charset"

This reverts commit 75e4a3195fe136ef2965fc0aafbdaebc7321f88a.

Reason for revert: After talking with the spec editor, we decided to
go back to the old behavior. This is safe because the change has not
been to beta/stable yet.

Original change's description:
> [XHR] Do not replace request content-type "utf-8" charset
> 
> The spec asks us to replace the charset to "UTF-8" only when it is not
> equal to "UTF-8" case insensitively.
> 
> https://xhr.spec.whatwg.org/#the-send()-method
> 
> """
> Otherwise, if the header whose name is a byte-case-insensitive match for
> `Content-Type` in author request headers has a value that is a valid
> MIME type, which has a `charset` parameter whose value is not a
> byte-case-insensitive match for encoding, and encoding is not null, then
> set all the `charset` parameters whose value is not a
> byte-case-insensitive match for encoding of that header’s value to
> encoding.
> """
> 
> The new behavior is aligned with Firefox:
> https://wpt.fyi/XMLHttpRequest/send-content-type-charset.htm
> 
> Bug: 651750
> Change-Id: Iff716e5ceece20e60bf8e69a73a980a4ea4fc6b9
> Reviewed-on: https://chromium-review.googlesource.com/773942
> Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
> Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
> Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#518203}

TBR=tyoshino@chromium.org,yhirano@chromium.org,foolip@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 651750
Change-Id: I72d4b8e9244dd7098748298800feecebb9a5b481
Reviewed-on: https://chromium-review.googlesource.com/792712
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519659}
2 files changed