Fix database serialization of job class names with Que

When passing a class constant as a job argument, Que’s stored procedures serialize it to JSON in the database as `{}`. This means that when the job (the ActiveJob “wrapper”) is deserialized from the database, it can’t find the original job class to run again.

Changing the Que adapter to serialize the job class as a string fixes this behaviour. This change makes the adapter consistent with other adapters too (which constantize a class string in their JobWrapper#perform methods.
Merge pull request #100 from timriley/fix-que-class-serialization

Fix database serialization of job class names with Que