The value of charset part for Content-Type HTTP header.

def charset(self) -> Optional[str]:
        """The value of charset part for Content-Type HTTP header."""
        raw = self._headers.get(hdrs.CONTENT_TYPE)  # type: ignore
        if self._stored_content_type != raw:
            self._parse_content_type(raw)
        return self._content_dict.get('charset')