code: private function createRequestOrderXML( OrderRequest $order )
      {
          $xmlWriter = new XMLWriter;
  
          $xmlWriter->openMemory();
          $xmlWriter->startDocument( '1.0', 'UTF-8' );
  
          $order->toXML( $xmlWriter );
  
          $xmlWriter->endDocument();
  
          return $xmlWriter->outputMemory( true );
      }
docs: @param  \RodrigoPedra\ClearSaleID\Entity\Request\Order $order
  
  @return string
  @throws \RodrigoPedra\ClearSaleID\Exception\RequiredFieldException