Set the content type as application/json

I copied off the jsonify function like you told me to check out from 
https://github.com/UWFlow/rmc/blob/master/server/api/api_util.py#L91
I removed the ApiJsonEncoder, wasn't sure if it was needed here. Let me know if I should add it back.
Please update this comment... this `jsonify` doesn't actually know how to encode Mongo ObjectIds and datetimes. :)