Do you have a reference for that setting the response format? The driver documentation doesn't mention it, only that you can specify the format of the request. There is an example response in JSON, but it's nothing like what I receive as a response. This is in the Tinkerpop docs. Janus seem to have removed driver development documentation from their release.

http://tinkerpop.apache.org/docs/3.2.5/dev/provider/#_graph_driver_provider_requirements