[fetch] Abort fetch upon unload document unload (#53)

The `fetch` spec doesn't define what a user agent should do when the document unloads. I expect the fetch request be aborted. This expectation seems reasonable, since it is similar to what the SW spec states: https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#on-fetch-request-algorithm:

> 18 .  ...
> If task is discarded or the script has been aborted by the termination of activeWorker, set handleFetchFailed to true.
> ...
> 21 . If handleFetchFailed is true, then:
>    1. Return a network error and run the following substeps in parallel.

> 22 . Else:
>    1. Return a response represented by response and run the following substeps in parallel.

Chrome's implementation already follows this behavior, Firefox does not (see https://bugzilla.mozilla.org/show_bug.cgi?id=1165237).

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/53