Ensures :attr:`subject` is a :class:`collections.Mapping` and contains *keys*, and no other keys.

def has_only_keys(self, keys):
        """
        Ensures :attr:`subject` is a :class:`collections.Mapping` and contains *keys*, and no other keys.
        """
        self.is_a(Mapping)
        self.contains_only(keys)
        return ChainInspector(self._subject)