add failing test for json-encoded attribute backend

Author: tkotosz

app/code/Magento/Eav/Test/Unit/Model/Entity/Attribute/Backend/JsonEncodedTest.php

@@ -82,6 +82,25 @@ public function testBeforeSave()
         $this->assertEquals(json_encode([1, 2, 3]), $product->getData('json_encoded'));
     }
 
+    /**
+     * Test before save handler with already encoded attribute value
+     */
+    public function testBeforeSaveWithAlreadyEncodedValue()
+    {
+        $product = new \Magento\Framework\DataObject(
+            [
+                'json_encoded' => [1, 2, 3]
+            ]
+        );
+
+        // save twice
+        $this->model->beforeSave($product);
+        $this->model->beforeSave($product);
+        
+        // check it is encoded only once
+        $this->assertEquals(json_encode([1, 2, 3]), $product->getData('json_encoded'));
+    }
+
     /**
      * Test after load handler
      */