Gets the Issuer of the Logout Response Message
        :return: The Issuer
        :rtype: string

def get_issuer(self):
        """
        Gets the Issuer of the Logout Response Message
        :return: The Issuer
        :rtype: string
        """
        issuer = None
        issuer_nodes = self.__query('/samlp:LogoutResponse/saml:Issuer')
        if len(issuer_nodes) == 1:
            issuer = OneLogin_Saml2_XML.element_text(issuer_nodes[0])
        return issuer