Fix issue #204 (auto-recovery related)

This patch fixes issue #204 for my test case.

Now I get completely stable reconnects with server-named queues and persistent (client-named) queues over multiple rabbitmq server restarts.

This line in queues.rb was not reliable working for me:

    @channel.queues.delete(old_name)

Moving the test inside channel.rb did help. Now there are no longer duplicates in @channel.queues and no redundant calls are made to on_recovery on the queue object.
You don't have to check for the value to clear it.