Stream with timeout should also yield None

Even when there is a low traffic, we know we didn't lose the
connection yet when ready_to_read indicates a signal from Twitter,
so we don't yield any timeout message and continue, but we should
at least yield None like in the non-blocking mode to allow
iterator's use to get the hand back and exit cleanly if required.

Completes 3df3413dc8
Ping @dkanygin #178