Check to see if we are dealing with an object with a toJson method

 - if we are dealing with something like new \Magento\Framework\DataObject() being used as a response object

Author: dmanners

lib/internal/Magento/Framework/Controller/Result/Json.php

@@ -59,7 +59,11 @@ public function __construct(
      */
     public function setData($data, $cycleCheck = false, $options = [])
     {
-        $this->json = $this->serializer->serialize($data);
+        if (is_object($data) && method_exists($data, 'toJson')) {
+            $this->json = $data->toJson();
+        } else {
+            $this->json = $this->serializer->serialize($data);
+        }
         return $this;
     }