[fetch] `user-agent` header control (#37)

Should `fetch()` set `user-agent` by default? Allow appending bytes? Allow replacing it? Allow it to be omitted?

See https://github.com/slightlyoff/ServiceWorker/issues/348#issuecomment-48568988 for context.

And why is it on the forbidden header list? It has been since forever, but is there strong rationale?

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https://github.com/whatwg/fetch/issues/37