Serialize to xml.

        :return string:

def serialize(self, content):
        """ Serialize to xml.

        :return string:

        """
        return self.xmldoc_tpl % (
            'true' if self.response.status_code == HTTP_200_OK else 'false',
            str(self.resource.api or ''),
            int(mktime(datetime.now().timetuple())),
            super(XMLTemplateEmitter, self).serialize(content)
        )