Don't busywait forever if Twitter hangs up on us.

Twitter will sometimes disconnect you while you're listening to a stream.  (In our experience this happens because the credentials you used are being used somewhere else.)  Currently that causes the stream iterator to spin forever reading zero bytes from the socket.  This patch causes it to return (ending the iterator) instead.

I am not sure what should happen in the case of timeout or non-blocking mode and did not attempt to address those.