more natural json for call service result

This patch enable to generate more natural json.
Natural means boolean value in json format.

Current version generates like this.
{id: "call_service:/add_two_ints:1", values: Object, result: "true", service: "/add_two_ints", op: "service_response"}

This patched version:
{id: "call_service:/add_two_ints:1", values: Object, result: true, service: "/add_two_ints", op: "service_response"}