perf: HTTP caching headers for /api/query_results [fixes #228]

this is not how we encode JSON data, see https://github.com/EverythingMe/redash/blob/master/redash/wsgi.py#L28

It will be best, if we can reuse the existing code (`json_representation`). I see that it takes headers as one param, can you check what the get method needs to return for it to use them?