Fix no ack producer

Several fixes:
1) Kafka does not send response message for *ProduceRequest* if *ack* option is 0. In this case connections queue (``self._requests``) can overflow with  pending futures. So I added optional key parameter ``without_resp``  to ``conn.send`` method. 
2) I missed a lot of  ``yield from``s in producers tests :)
3) typo fix in ``__repr__``