Add suppport to custom routing_key if passing a Queue

I need to use routing keys in my application - to avoid using multiple queues -, but dont want to it be fully asynchronous (eg.: I dont want to use Consumer class directly), so, i made this little modification to the Simple interface: When passing a `kombu.entity.Queue` class, it get the routing key from the Queue and use in the producer, directly.

This avoids the possible issue of, for example, when passing a `kombu.entity.Queue` with a custom Routing Key, the producer starts sending the messages to the wrong routing key, too. =)