code: def on_header(self, name: bytes, value: bytes) -> None:
          
          name_ = decode_bytes(name).casefold()
          val = decode_bytes(value)
          if name_ == "cookie":
              # 加载上次的 cookie
              self._cookies.load(val)
          if name_ in self._headers:
              # 多个相同的 header
              old = self._headers[name_]
              if isinstance(old, list):
                  old.append(val)
              else:
                  self._headers[name_] = [old, val]
          else:
              self._headers[name_] = val
docs: header 回调