set time of published at in learning_object

app/services/learning_object_publisher.rb

@@ -29,6 +29,7 @@ class LearningObjectPublisher
     return true if learning_object.published?
 
     learning_object.state = LearningObject.states[:published]
+    learning_object.published_at = Time.now
     ThumbnailGenerateWorker.perform_async learning_object.id, learning_object.link if learning_object.link?
     learning_object.save
   end