endpoints-proto-datastore: Issue 52 hack

class MyModel(EndpointsModel):
  _message_fields_schema = ('id', ...)

  # ...

  @EndpointsAliasProperty(setter=EndpointsModel.IdSet,
                          property_type=messages.IntegerField,
                          variant=messages.Variant.INT32)
  def id(self):
    return super(MyModel, self).id

# OR
class MyModel(EndpointsModel):
  _message_fields_schema = ('id', ...)

  # ...
MyModel.id._variant = messages.Variant.INT32