[whatwg/xhr] Fix send() charset overriding to use new MIME type infrastructure (#188)

This is a known issue but I don't see a tracking bug. Basically,

> 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. 

should be done in terms of MIME Type parsing and serialization.

I'm especially wondering what to do if parsing fails.

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