[ServiceWorker] Cache and Request objects (#624)

It's not entirely clear to me how this works right now. E.g. does `keys()` always return clones? (The text for `keys()` is most certainly wrong.)

I think that perhaps we should introduce `RequestKey` or some such and introduce a mixin that `Request` and `RequestKey` can share. And ways to create a `RequestKey` from a `Request` (and vice versa?). That way caches never have to deal with request that has an associated body.

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/624