Fix bad logic on standard Redis failed queue adapter

There were some checks for the queue name in this adapter, in place to
provide a similar API to the multi-queue backend, but with assertions
that we were operating on the right queue.

Problem is: the logic was backwards! D'oh!

This centralizes the check in a method with the correct logic.
Merge pull request #889 from tarcieri/fix_failed_queue

Fix bad logic on standard Redis failed queue adapter