Exception handling `session_get`

[da4089]

From the agent log:

2018/04/19-11:35:07.623 asyncio ERROR  Exception in callback FixToolAgent.readable(<socket.socke....0.1', 60084)>)
handle: <Handle FixToolAgent.readable(<socket.socke....0.1', 60084)>)>
Traceback (most recent call last):
  File "/usr/lib/python3.6/asyncio/events.py", line 127, in _run
    self._callback(*self._args)
  File "/usr/lib/python3.6/site-packages/fixtool/agent.py", line 480, in readable
    self.handle_request(control_session, message)
  File "/usr/lib/python3.6/site-packages/fixtool/agent.py", line 543, in handle_request
    self.handle_session_get(client, message)
  File "/usr/lib/python3.6/site-packages/fixtool/agent.py", line 946, in handle_session_get
    control.send(response.to_json().encode())
  File "/usr/lib/python3.6/site-packages/fixtool/message.py", line 819, in to_json
    "payload": self.payload})
  File "/usr/lib/python3.6/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "/usr/lib/python3.6/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python3.6/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/usr/lib/python3.6/json/encoder.py", line 180, in default
    o.__class__.__name__)
TypeError: Object of type 'bytes' is not JSON serializable