Fix problem when queue is not created after being deleted and redeclared

A problem was reported on the ruby-amqp mailing list where a queue was declared, then deleted and finally redeclared. After the last redeclaration the queue was not being recreated because it was still cached in the Bunny::Channel hash @queues.

This patch is to make sure that when exchanges and queues are deleted they are removed from Bunny::Channel hashes @exchanges and @queues.