获取 header

def get(self, name: str) -> Union[None, str, List[str]]:
        """
        获取 header
        """
        name = name.casefold()
        if name == "referer" or name == "referrer":
            if "referrer" in self._headers:
                return self._headers["referrer"]
            elif "referer" in self._headers:
                return self._headers["referer"]
            else:
                return None
        elif name in self._headers:
            return self._headers[name]
        else:
            return None