Adds a new value for the given key.

def add(self, name: str, value: str) -> None:
        """Adds a new value for the given key."""
        norm_name = _normalized_headers[name]
        self._last_key = norm_name
        if norm_name in self:
            self._dict[norm_name] = (
                native_str(self[norm_name]) + "," + native_str(value)
            )
            self._as_list[norm_name].append(value)
        else:
            self[norm_name] = value