after deleting, get_thing_shadow will raise ResourceNotFound.
        But version of the shadow keep increasing...

def delete_thing_shadow(self, thing_name):
        """after deleting, get_thing_shadow will raise ResourceNotFound.
        But version of the shadow keep increasing...
        """
        thing = iot_backends[self.region_name].describe_thing(thing_name)
        if thing.thing_shadow is None:
            raise ResourceNotFoundException()
        payload = None
        new_shadow = FakeShadow.create_from_previous_version(thing.thing_shadow, payload)
        thing.thing_shadow = new_shadow
        return thing.thing_shadow