Gets the Issuer of the Logout Request Message
        :param request: Logout Request Message
        :type request: string|DOMDocument
        :return: The Issuer
        :rtype: string

def get_issuer(request):
        """
        Gets the Issuer of the Logout Request Message
        :param request: Logout Request Message
        :type request: string|DOMDocument
        :return: The Issuer
        :rtype: string
        """

        elem = OneLogin_Saml2_XML.to_etree(request)
        issuer = None
        issuer_nodes = OneLogin_Saml2_XML.query(elem, '/samlp:LogoutRequest/saml:Issuer')
        if len(issuer_nodes) == 1:
            issuer = OneLogin_Saml2_XML.element_text(issuer_nodes[0])
        return issuer