Returns an iterable of all (name, value) pairs.

        If a header has multiple values, multiple pairs will be
        returned with the same name.

def get_all(self) -> Iterable[Tuple[str, str]]:
        """Returns an iterable of all (name, value) pairs.

        If a header has multiple values, multiple pairs will be
        returned with the same name.
        """
        for name, values in self._as_list.items():
            for value in values:
                yield (name, value)