Dump Custom objects with slots

Sending request to server results in  exception:
```
Traceback (most recent call last):
  File "C:\Utilities\eclipse\plugins\org.python.pydev_3.5.0.201405201709\pysrc\pydevd.py", line 1845, in <module>
    debugger.run(setup['file'], None, None)
  File "C:\Utilities\eclipse\plugins\org.python.pydev_3.5.0.201405201709\pysrc\pydevd.py", line 1373, in run
    pydev_imports.execfile(file, globals, locals)  # execute the script
  File "D:\Data\Workspace\test_samples_utilities\jsonrpc_test.py", line 15, in <module>
    auth = s.ana_rpc("", "login", auth_login_encode("guest", "guest"))["result"]
  File "build\bdist.win32\egg\jsonrpclib\jsonrpc.py", line 446, in __call__
  File "build\bdist.win32\egg\jsonrpclib\jsonrpc.py", line 349, in _request
  File "build\bdist.win32\egg\jsonrpclib\jsonrpc.py", line 906, in check_for_errors
jsonrpclib.jsonrpc.ProtocolError: (-32603, u"<type 'exceptions.AttributeError'>:'MessagingObject' object has no attribute '__dict__'")
```

Looking into MessagingObject I found out that it is an object which has defined \_\_slots\_\_ and thus it does not have a \_\_dict\_\_.

I made some modifications to get object with slots to response.