Ensure that the encode and decode are symmetric

It's difficult to build a non brittle `redis_key` function when
sometimes receives a hash of symbols and sometimes receives a
hash of strings (a decoded JSON from Redis).
https://github.com/resque/resque-loner/blob/71c86abaf759b6f0a0353e533c4823ea3e6c7855/test/resque_failure_redis_test.rb#L17 :eyes: