Updates a custom field belonging to this list.

def update_custom_field(self, custom_field_key, field_name,
                            visible_in_preference_center):
        """Updates a custom field belonging to this list."""
        custom_field_key = quote(custom_field_key, '')
        body = {
            "FieldName": field_name,
            "VisibleInPreferenceCenter": visible_in_preference_center}
        response = self._put(self.uri_for("customfields/%s" %
                                          custom_field_key), json.dumps(body))
        return json_to_py(response)