Re: [fetch] Support full-duplex HTTP streaming (#229)

Also:

- Finish speccing request body streams
- Make sure there's nothing in the spec that breaks this (e.g. buffering until the request body is finished) or makes assumptions contrary to it (e.g. assumes that response body bytes are received after request body bytes are finished uploading).

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https://github.com/whatwg/fetch/issues/229#issuecomment-191433251