Apply some small optimizations to QueueingConsumer#deliver

Avoid calling methods multiple times, basically.
Add a test for the handling of empty batches

And remove the comment.
Publish persistent messages by default
Avoid creating a hash on each call to Publisher#publish

The method doesn’t actually use the options passed to it, but until we do a backwards-incompatible cleanup it needs to remain – we just don’t need it to be a hash.