Clears an outgoing header, undoing a previous `set_header` call.

        Note that this method does not apply to multi-valued headers
        set by `add_header`.

def clear_header(self, name: str) -> None:
        """Clears an outgoing header, undoing a previous `set_header` call.

        Note that this method does not apply to multi-valued headers
        set by `add_header`.
        """
        if name in self._headers:
            del self._headers[name]