Auto merge of #25359 - pshaughn:auto_ct_header, r=jdm

Autocomputed content-type header now reaches net request

The spec expects that for a DOM Request r, r.headers and r.request can actually refer to the same header list in RAM, with changes to one affecting the other. This is mostly unobservable, but it happens to come up at the point in the Request constructor that auto-infers a content type from the body, so now after inferring the content type it injects it into both header lists instead of one.
Remaining test failures are due to the way Hyper crates normalize semicolons in MIME types, and an actually separate problem about content-lengths that I haven't sniffed out yet.

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