Call #to_liquid before calling #to_json in jsonify filter.

I was working on https://github.com/WhiteHouse/playbook/pull/7#issuecomment-51852244 and remembered that the `jsonify` filter didn't call `#to_liquid` before turning it into json.

Another option is to alias `#to_json` to `#to_liquid` in `Document`, and `Convertible`.