Payload StringBody::NextPayload(bool /*free_previous*/) {
  if (index_ == 0) {
    index_ = 1;
    return { boost::asio::buffer(data_) };
  }
  return {};
}
Payload StringBody::NextPayload(bool  ) {
  if (index_ == 0) {
    index_ = 1;
    return { boost::asio::buffer(data_) };
  }
  return {};
}