Fix for pickle.loads

[wojcikstefan]

pickle.loads fails on documents without any changed fields:

    body = pickle.loads(base64.b64decode(task['body']))
  File "/usr/lib/python2.7/pickle.py", line 1382, in loads
    return Unpickler(file).load()
  File "/usr/lib/python2.7/pickle.py", line 858, in load
    dispatch[key](self)
  File "/usr/lib/python2.7/pickle.py", line 1217, in load_build
    setstate(state)
  File "/home/ubuntu/closeio/venv/src/mongoengine/mongoengine/base/document.py", line 155, in __setstate__
    setattr(self, k, data[k])
KeyError: '_changed_fields'

This PR fixes it.

[amcgregor]

+1

[rozza]

@wojcikstefan thanks - thats embarrassing as I added that for __get_state__ :(