message: Fix IndexUpdate::ToString
diff: @@ -728,7 +728,7 @@ void IndexUpdate::Merge(IndexUpdate&& update) {
  std::string IndexUpdate::ToString() {
  rapidjson::StringBuffer output;
  - rapidjson::Writer<rapidjson::StringBuffer> writer;
  + rapidjson::Writer<rapidjson::StringBuffer> writer(output);
  JsonWriter json_writer(&writer);
  IndexUpdate& update = *this;
  Reflect(json_writer, update);
  