JSON/CSV issues with display_column?

The JSON responder has an issue with the `#display_column?` feature. Headers for hidden columns aren't removed, but the value is, so the columns and values don't match up after the headers and values are zipped into a hash (unless the hidden column is at the end).

CSV, on the other hand, doesn't seem to be hooked into the `display_column?` logic at all since the responder uses `#raw_results`.