header 回调

def on_header(self, name: bytes, value: bytes) -> None:
        """
        header 回调
        """
        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