void FormBody::Dump(std::ostream& os, const std::string& prefix) const {
  for (auto& part : parts_) {
    os << prefix << "--" << boundary_ << std::endl;
    part->Dump(os, prefix);
  }
  os << prefix << "--" << boundary_ << "--" << std::endl;
}
void FormBody::Dump(std::ostream& os, const std::string& prefix) const {
  for (auto& part : parts_) {
    os << prefix << "--" << boundary_ << std::endl;
    part->Dump(os, prefix);
  }
  os << prefix << "--" << boundary_ << "--" << std::endl;
}