获取 charset

def charset(self) -> Optional[str]:
        """
        获取 charset
        """
        type_str = cast(str, self.get("Content-Type"))
        if type_str is None or "charset" not in type_str:
            return None
        for i in type_str.split(";"):
            item = i.strip()
            if item.startswith("charset"):
                return item.split("=")[1].strip()
        return None