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Streams 1de3b9aac860

Set shuttingDown at the beginning of pipe shutdown In the implementation of pipeTo(), setting _shuttingDown_ after flushing unwritten reads makes it difficult for implementations to obey the requirement that "Shutdown must stop activity". Generally it means they have to track separately whether they've started to shutdown, making the _shuttingDown_ variable redundant. The exact behaviour while unwritten reads are being flushed may also be ambiguous. Change to set _shuttingDown_ at the beginning of the _Shutdown with an action_ and _Shutdown_ steps. No changes to reference implementation as it already works this way. Fixes #934.

Adam Rice