Updates the options of a multi-optioned custom field on this list.

def update_custom_field_options(self, custom_field_key, new_options,
                                    keep_existing_options):
        """Updates the options of a multi-optioned custom field on this list."""
        custom_field_key = quote(custom_field_key, '')
        body = {
            "Options": new_options,
            "KeepExistingOptions": keep_existing_options}
        response = self._put(self.uri_for(
            "customfields/%s/options" % custom_field_key), json.dumps(body))