[streams] Make stream readers look closed when released (#260)

In https://github.com/whatwg/streams/issues/241#issuecomment-68834017 @tyoshino proposed making a stream reader look "closed" once it is released. Right now it just passes through `ready`/`state`/`closed` to the underlying stream, although `read()` and `cancel()` fail. I think making it look closed instead of being a pass-through would be better separation of what's going on.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/260