make sure the job data is passed unchanged through redis/lua

If the job data JSON contains empty arrays they are mangled by
the decoding to a lua table. This commit stores and returns the raw
job data without processing it with lua/cjson.

This pull request belongs to seomoz/qless#73 and tries to be a fix for it.

The change to `get.lua` qualifies as a hack and I would be happy for alternative suggestions.

The way I see it there are only two options:

1. Use `cjson.encode` to create the JSON response and squeeze in the raw job_data afterwards (my current approach).
2. Create the whole JSON container manually without cjson which involves quite a lot of string manipulation.