Fix wrong format error DHL shipping label

Fix "The response is in wrong format." error while generating dhl shipping label. When product name has unicode characters substr() is sending � in <PieceContents> causing erros dhl syntax errors.

Author: vpsnak

app/code/Magento/Dhl/Model/Carrier.php

@@ -1748,7 +1748,7 @@ protected function _shipmentDetails($xml, $rawRequest, $originRegion = '')
             foreach ($package['items'] as $item) {
                 $content[] = $item['name'];
             }
-            $nodePiece->addChild('PieceContents', substr(implode(',', $content), 0, 34));
+            $nodePiece->addChild('PieceContents', $this->string->substr(implode(',', $content), 0, 34));
         }
 
         $nodeShipmentDetails->addChild('Weight', sprintf('%.3f', $rawRequest->getPackageWeight()));