the return value from the server 

When the client have got `None` value from the server, `AssertionError` is raised.

The following is a sample server-side and client-side code that causes this problem.

## server

    import msgpackrpc

    class Server:
        def sum(self, x, y):
            return x + y
        def something_no_return_func(self):
            # somthing do ..
            x = 1
            # return None

    server = msgpackrpc.Server(Server())
    server.listen(msgpackrpc.Address("localhost", 18800))
    server.start()

## client

    import msgpackrpc

    client = msgpackrpc.Client(msgpackrpc.Address("localhost", 18800))
    result = client.call('sum', 1, 2)
    result = client.call('something_no_return_func')


## error

    $ python client.py
    ERROR:root:Uncaught exception, closing connection.
    Traceback (most recent call last):
      File "/Users/kazupon/.virtualenvs/kt-msgpack/lib/python2.7/site-packages/tornado/iostream.py", line 304, in wrapper
        callback(*args)
      File "/Users/kazupon/.virtualenvs/kt-msgpack/lib/python2.7/site-packages/msgpackrpc/transport/tcp.py", line 26, in on_read
        self.on_message(message)
      File "/Users/kazupon/.virtualenvs/kt-msgpack/lib/python2.7/site-packages/msgpackrpc/transport/tcp.py", line 37, in on_message
        self.on_response(message[1], message[2], message[3])
      File "/Users/kazupon/.virtualenvs/kt-msgpack/lib/python2.7/site-packages/msgpackrpc/transport/tcp.py", line 73, in on_response
        self._transport._session.on_response(msgid, error, result)
      File "/Users/kazupon/.virtualenvs/kt-msgpack/lib/python2.7/site-packages/msgpackrpc/session.py", line 99, in on_response
        future.set_result(result)
      File "/Users/kazupon/.virtualenvs/kt-msgpack/lib/python2.7/site-packages/msgpackrpc/future.py", line 47, in set_result
        assert result is not None
    AssertionError
    ERROR:root:Exception in callback <tornado.stack_context._StackContextWrapper object at 0x101944788>
    Traceback (most recent call last):
      File "/Users/kazupon/.virtualenvs/kt-msgpack/lib/python2.7/site-packages/tornado/ioloop.py", line 399, in _run_callback
        callback()
      File "/Users/kazupon/.virtualenvs/kt-msgpack/lib/python2.7/site-packages/tornado/iostream.py", line 304, in wrapper
        callback(*args)
      File "/Users/kazupon/.virtualenvs/kt-msgpack/lib/python2.7/site-packages/msgpackrpc/transport/tcp.py", line 26, in on_read
        self.on_message(message)
      File "/Users/kazupon/.virtualenvs/kt-msgpack/lib/python2.7/site-packages/msgpackrpc/transport/tcp.py", line 37, in on_message
        self.on_response(message[1], message[2], message[3])
      File "/Users/kazupon/.virtualenvs/kt-msgpack/lib/python2.7/site-packages/msgpackrpc/transport/tcp.py", line 73, in on_response
        self._transport._session.on_response(msgid, error, result)
      File "/Users/kazupon/.virtualenvs/kt-msgpack/lib/python2.7/site-packages/msgpackrpc/session.py", line 99, in on_response
        future.set_result(result)
      File "/Users/kazupon/.virtualenvs/kt-msgpack/lib/python2.7/site-packages/msgpackrpc/future.py", line 47, in set_result
        assert result is not None
    AssertionError

I fixed the this problem.
This value may be received from a server that is different from other language implementation.
Please review :-)