AMQP backend: Don't use different values for `auto_delete`.
AMQP backend: Don't use different values for `auto_delete`.

This bug became visible with RabbitMQ 1.8.0, which no longer
allows conflicting declarations for the auto_delete and durable settings.

frac/celery,mitsuhiko/celery,ask/celery,cbrepo/celery,cbrepo/celery,ask/celery,frac/celery,WoLpH/celery,WoLpH/celery,mitsuhiko/celery