Fix: avoid losing message meta-data because of weak type evaluation

I got frustrated when I set my `AMQPMessage` `priority` meta-data to `0` and not having it back when consuming my message in my worker.

As the `basic_consume` can handle both positive and negative, I figured out that message could too. Even I know message priority for for now just a placeholder.

Maybe there is an explicit reason for this but maybe not so here's my contribution.