code: def get_issuer(self):
          
          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
docs: Gets the Issuer of the Logout Response Message
          :return: The Issuer
          :rtype: string