CRAM-MD5 mechanism does not work in python-client

CRAM-MD5 mechanism does not work in python client, see e.g.:

$ qpid-stat -q -b guest/guest@localhost:5672 --sasl-mechanism=CRAM-MD5
Failed: ConnectionError - connection-forced: Authentication failed(320)
$ qpid-stat -q -b guest/guest@localhost:5672 --sasl-mechanism=DIGEST-MD5
Queues
queue dur autoDel excl msg msgIn msgOut bytes bytesIn bytesOut cons bind
..
$

The root cause is that Python client sends initial SASL response in ConnectionStartOk AMQP frame even in case it is empty. In such case, it should not set response.