Add default behavior to Quote\Address\TotalFactory

The factory is used like a default generated factory in TotalsReader but
needed to pass the class name to create(). This violates the POLA. Also,
the return type hint is adjusted now (Total is no subtype of
Total\AbstractTotal)

Author: schmengler

app/code/Magento/Quote/Model/Quote/Address/TotalFactory.php

@@ -33,9 +33,9 @@ public function __construct(\Magento\Framework\ObjectManagerInterface $objManage
      *
      * @param string $instanceName
      * @param array $data
-     * @return \Magento\Quote\Model\Quote\Address\Total\AbstractTotal
+     * @return Total\AbstractTotal|Total
      */
-    public function create($instanceName, array $data = [])
+    public function create($instanceName = Total::class, array $data = [])
     {
         return $this->_objectManager->create($instanceName, $data);
     }

app/code/Magento/Quote/Model/Quote/TotalsReader.php

@@ -40,7 +40,7 @@ public function __construct(
     public function fetch(\Magento\Quote\Model\Quote $quote, array $total)
     {
         $output = [];
-        $total = $this->totalFactory->create(\Magento\Quote\Model\Quote\Address\Total::class)->setData($total);
+        $total = $this->totalFactory->create()->setData($total);
         /** @var ReaderInterface $reader */
         foreach ($this->collectorList->getCollectors($quote->getStoreId()) as $reader) {
             $data = $reader->fetch($quote, $total);
@@ -73,14 +73,12 @@ protected function convert($total)
         if (count(array_column($total, 'code')) > 0) {
             $totals = [];
             foreach ($total as $item) {
-                $totals[] = $this->totalFactory->create(
-                    \Magento\Quote\Model\Quote\Address\Total::class
-                )->setData($item);
+                $totals[] = $this->totalFactory->create()->setData($item);
             }
             return $totals;
         }
 
-        return $this->totalFactory->create(\Magento\Quote\Model\Quote\Address\Total::class)->setData($total);
+        return $this->totalFactory->create()->setData($total);
     }
 
     /**