stream: Use push() for Transform._output()

This also slightly changes the semantics, in that a 'readable'
event may be triggered by the first write() call, even if a
user has not yet called read().

This happens because the Transform _write() handler is calling
read(0) to start the flow of data.  Technically, the new behavior
is more 'correct', since it is more in line with the semantics
of the 'readable' event in other streams.