Introduce Envelope#toString

Contributed by Michał Kiędyś in
https://github.com/rabbitmq/rabbitmq-java-client/pull/8
Introduce GetResponse#toString
Cover all built in exchange types in testUnbind()
Merge bug26015
Merge bug 26014
stable to default
merge default into bug14587
Let BrokerTestCase#tearDown handle connection termination where possible