[ServiceWorker] Cache should sanitize the stored Request in put/add/addAll (#664)

There are some values on the Request object that should not be persisted.  Fetch handles this by sanitizing the input Request.  See step 4 from here:

  https://fetch.spec.whatwg.org/#dom-request

@annevk suggests Cache should have a similar sanitize step.

The attributes I know about that should not be stored are:

* context (Cache should always produce a context of "fetch")
* synchronous flag

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/664