Generate an XML element with client auth data populated.

def client_auth(self):
        """Generate an XML element with client auth data populated."""
        if not self._client_auth:
            self._client_auth = E.Element('merchantAuthentication')
            E.SubElement(self._client_auth, 'name').text = self.config.login_id
            E.SubElement(self._client_auth, 'transactionKey').text = self.config.transaction_key
        return self._client_auth