[fetch] Service worker request body reads need to invoke callbacks (#87)

The HTTP-network fetch algorithm currently queues "process request body" tasks, which also need to be queued by HTTP fetch as otherwise the service worker doing `event.request.text()` is not defined.

Similarly, text for response updating probably needs to be included.

Both of these changes also prepare us for streams.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/87