Error behavior in SONManipulator

There is bug with SONManipulator optimization here: https://github.com/mongodb/mongo-python-driver/blob/master/pymongo/database.py#L95

Due to this bug:

• we cannot create manipulator that isn't inherited from SONManipulator class. That's not so good.
• we cannot create two (or more) level manipulators inheritance hierarhy. For example this wouldn't work correct:

  from pymongo.son_manipulator import SONManipulator

  class ParentManipulator(SONManipulator):

  def transform_incoming(self, son, collection):

  son['collection_name'] = collection.name

  return son

  def transform_outgoing(self, son, collection):

  if 'collection_name' in son:

  del son['collection_name']

  return son

  class ChildManipulator(ParentManipulator):

  """

  It should work like ParentManipulator, but it wouldn't work at call :(

  """

  pass