[whatwg/fetch] Figure out if request header mutation is possible (#391)

There's a number of things that would be nice to mutate in a request as it passes through a service worker, but a particular one that comes up often and might be the easiest is adding a header.

Adding headers seems fine when request mode is "same-origin" or "cors", but harder when it is "navigate" or "no-cors" (see also #380).

For "navigate" and "no-cors" we could maybe tag an additional flag to the added headers (making the map a little more complicated, which I guess we could do after we fix some other issues with headers) and drop the added headers upon encountering a (cross-origin?) redirect.

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