Ensure that the Redis and RedisMultiQueue failure types correctly extend Resque::Failure::Each

Ensure that the Redis and RedisMultiQueue failure types correctly extend (not include) Resque::Failure::Each. This fixes a breaking change introduced
in 890ca9f.

Additionally, the refactoring also changed the behavior of ::each such
that if there were a single failure, the given block with yield to each
attribute of the failure rather than just the single failure.

Fixes #992