[whatwg/fetch] CORS request-header Content-Type check should be on a combined value (#748)

In particular if there are multiple `Content-Type` headers this check should most likely fail.

This is probably best addressed after #736.

This also affects the `Headers` API... Relates to https://github.com/whatwg/mimesniff/issues/30 as well, but can probably be addressed separately as request `Content-Type` can be much stricter than response `Content-Type`...

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