Forward arguments in JSONResponseMixin

This patch makes possible to forward arguments from render_json_response to json.dumps.

In my case, I need to output with indent=2. It is obviously possible to override render_json_response() completely, but I think that's more elegant to take an argument and give it to json.dumps.

Currently, ensure_ascii is set to True. This patch makes possible to set it to False even if I've no idea about why someone would do such a thing.