Extracts a node from the DOMDocument (Logout Response Menssage)
        :param query: Xpath Expresion
        :type query: string
        :return: The queried node
        :rtype: DOMNodeList

def __query(self, query):
        """
        Extracts a node from the DOMDocument (Logout Response Menssage)
        :param query: Xpath Expresion
        :type query: string
        :return: The queried node
        :rtype: DOMNodeList
        """
        # Switch to lxml for querying
        xml = self.document.toxml()
        return OneLogin_Saml2_Utils.query(fromstring(xml, forbid_dtd=True), query)