Call download rich menu image API.

        https://developers.line.me/en/docs/messaging-api/reference/#download-rich-menu-image

        :param str rich_menu_id: ID of the rich menu with the image to be downloaded
        :param timeout: (optional) How long to wait for the server
            to send data before giving up, as a float,
            or a (connect timeout, read timeout) float tuple.
            Default is self.http_client.timeout
        :type timeout: float | tuple(float, float)
        :rtype: :py:class:`linebot.models.responses.Content`
        :return: Content instance

def get_rich_menu_image(self, rich_menu_id, timeout=None):
        """Call download rich menu image API.

        https://developers.line.me/en/docs/messaging-api/reference/#download-rich-menu-image

        :param str rich_menu_id: ID of the rich menu with the image to be downloaded
        :param timeout: (optional) How long to wait for the server
            to send data before giving up, as a float,
            or a (connect timeout, read timeout) float tuple.
            Default is self.http_client.timeout
        :type timeout: float | tuple(float, float)
        :rtype: :py:class:`linebot.models.responses.Content`
        :return: Content instance
        """
        response = self._get(
            '/v2/bot/richmenu/{rich_menu_id}/content'.format(rich_menu_id=rich_menu_id),
            timeout=timeout
        )

        return Content(response)