Re: [ServiceWorker] should respondWith(undefined) be treated the same as not calling respondWith() (#766)

Undefined is returned to `respondWith` if all of the following is true:

* The original `fetch(event.request)` failed
* There's no match in the cache
* The request does not have `text/html` in its `Accept` header (this is sniffing for a navigation-like request)

Returning undefined seems reasonable here, and should be treated as a network failure.

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