Serialize to XML.

        :return string: serialized XML

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

        :return string: serialized XML

        """
        worker = XMLSerializer(
            scheme=self.resource,
            format=self.resource._meta.emit_format,
            options=self.resource._meta.emit_options,
            **self.resource._meta.emit_models
        )
        return self.xmldoc_tpl % (
            'true' if not self.response.error else 'false',
            str(self.resource.api or ''),
            int(mktime(datetime.now().timetuple())),
            worker.serialize(content)
        )