Deserializes the IonObject going into the JSON request

An IonObject was getting serialized with the JSON encoder, this patch
applies an IonObject deserializer before the json encoding to patch the
test:
ion/agents/instrument/test/test_gateway_to_instrument_agent.py:TestInstrumentAgentViaGateway.test_get_set_agent
Merge pull request #459 from lukecampbell/agentgateway

Deserializes the IonObject going into the JSON request