Registers the given representer class with this registry, using
        its MIME content type as the key.

def register_representer_class(self, representer_class):
        """
        Registers the given representer class with this registry, using
        its MIME content type as the key.
        """
        if representer_class in self.__rpr_classes.values():
            raise ValueError('The representer class "%s" has already been '
                             'registered.' % representer_class)
        self.__rpr_classes[representer_class.content_type] = representer_class
        if issubclass(representer_class, MappingResourceRepresenter):
            # Create and hold a mapping registry for the registered resource
            # representer class.
            mp_reg = representer_class.make_mapping_registry()
            self.__mp_regs[representer_class.content_type] = mp_reg