Set models.DEFAULT_KEYSPACE when calling set_session()
Set models.DEFAULT_KEYSPACE when calling set_session()

Sets the default keyspace for cqlengine models when passing in an
existing session to cqlengine.connection.set_session() if the session's
keyspace is set and models.DEFAULT_KEYSPACE isn't. Without this, the
user will see an error when trying to perform Cassandra interactions
with a Model instance like "Keyspace none does not exist" unless they
explicitly set models.DEFAULT_KEYSPACE themselves ahead of time or have
a __keyspace__ attribute set on the model.
Also no longer raising an exception in cqlengine.connection.setup() if
default_keyspace isn't set since individual models might still have
__keyspace__ set.

beobal/python-driver,stef1927/python-driver,coldeasy/python-driver,mambocab/python-driver,markflorisson/python-driver,mambocab/python-driver,thobbs/python-driver,thelastpickle/python-driver,kishkaru/python-driver,mobify/python-driver,markflorisson/python-driver,thelastpickle/python-driver,beobal/python-driver,tempbottle/python-driver,bbirand/python-driver,jregovic/python-driver,jfelectron/python-driver,coldeasy/python-driver,mobify/python-driver,vipjml/python-driver,yi719/python-driver,datastax/python-driver,stef1927/python-driver,yi719/python-driver,jfelectron/python-driver,jregovic/python-driver,bbirand/python-driver,kishkaru/python-driver,tempbottle/python-driver,datastax/python-driver,thobbs/python-driver,vipjml/python-driver