[fetch] consider different default context value than "fetch" for new Request() (#29)

Currently `new Request()` will get a context of "fetch" even though it may not be used with the `fetch()` function.  Perhaps we should distinguish this case by using a different default context value.  Maybe empty string or "synthesized".

This is related to "what should we do with context" in Cache:

  https://github.com/slightlyoff/ServiceWorker/issues/664

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/29