Allow the serializer to be an optional injection

Author: dmanners

app/code/Magento/Braintree/Gateway/Response/VaultDetailsHandler.php

@@ -60,13 +60,14 @@ public function __construct(
         OrderPaymentExtensionInterfaceFactory $paymentExtensionFactory,
         Config $config,
         SubjectReader $subjectReader,
-        \Magento\Framework\Serialize\SerializerInterface $serializer
+        \Magento\Framework\Serialize\SerializerInterface $serializer = null
     ) {
         $this->paymentTokenFactory = $paymentTokenFactory;
         $this->paymentExtensionFactory = $paymentExtensionFactory;
         $this->config = $config;
         $this->subjectReader = $subjectReader;
-        $this->serializer = $serializer;
+        $this->serializer = $serializer ?: \Magento\Framework\App\ObjectManager::getInstance()
+            ->get(\Magento\Framework\Serialize\SerializerInterface::class);
     }
 
     /**