code: def flush_headers(self, sync: bool = False) -> None:
          
          if self._headers_sent:
              return
          self._headers_sent = True
          self.handel_default()
          self.write(
              b"HTTP/%s %d %s\r\n" % (
                  encode_str(self._version),
                  self._status,
                  self._message,
              ),
              sync,
          )
          for name, value in self._headers.items():
              name_byte = encode_str(name)
              if isinstance(value, list):
                  for val in value:
                      self.write(
                          b"%s: %s\r\n" % (
                              name_byte,
                              encode_str(val),
                          ),
                          sync,
                      )
              else:
                  val = value
                  self.write(
                      b"%s: %s\r\n" % (
                          name_byte,
                          encode_str(value),
                      ),
                      sync,
                  )
          self.write(b"\r\n", sync)
docs: 通过异步写入 header