Attempt at making pull() be called every time something is enqueued or d...

...equeued in RS. See #189.
I wondered if we can check if we can write a test to cover this `stream._state === 'closed'` check. This is required to detect `rs.cancel()` was called or not, right?

It seems we never hit this since all the callers of `CallReadableStreamPull()` have `stream._state === 'closed'` check before it so far.

Anyway, just wondering. I agree it's good to keep this check.