fix comparison between serialized items and data

Coordinator could add push multiple copies of the same object because the comparison was between serialized data vs unserialized list of existing items in the queue.

This fixes that, by comparing serialized data against list of serialized items.

Need this to use in a tasker-email PR

@tylermercier 
`@redis.rpush(@queue_name, data) unless items.include?(item)` would have fixed this too. Comparing the serialized versions will have more consistent results though.