[fetch] Clear response body under certain conditions (#31)

>From @wanderview:

> Looking at the gecko code it appears that there are many cases where the content is explicitly ignored regardless of what the server does:
>
> * HEAD method
> * CONNECT method
> * Responses with status codes 101, 204, 205, 304
>
> See mNoContent in the code here:
>
> https://dxr.mozilla.org/mozilla-central/source/netwerk/protocol/http/nsHttpTransaction.cpp#1523

See also https://github.com/slightlyoff/ServiceWorker/issues/655 and https://github.com/w3c/web-platform-tests/pull/1697

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https://github.com/whatwg/fetch/issues/31