Add a MAC token to the request authorization header.

        Warning: MAC token support is experimental as the spec is not yet stable.

def _add_mac_token(self, uri, http_method='GET', body=None,
                       headers=None, token_placement=AUTH_HEADER, ext=None, **kwargs):
        """Add a MAC token to the request authorization header.

        Warning: MAC token support is experimental as the spec is not yet stable.
        """
        if token_placement != AUTH_HEADER:
            raise ValueError("Invalid token placement.")

        headers = tokens.prepare_mac_header(self.access_token, uri,
                                            self.mac_key, http_method, headers=headers, body=body, ext=ext,
                                            hash_algorithm=self.mac_algorithm, **kwargs)
        return uri, headers, body