[DDC-1861] Fix UnitOfWork#doMerge()

lib/Doctrine/ORM/UnitOfWork.php

@@ -1655,7 +1655,7 @@ class UnitOfWork implements PropertyChangedListener
         $oid = spl_object_hash($entity);
 
         if (isset($visited[$oid])) {
-            return; // Prevent infinite recursion
+            return $visited[$oid]; // Prevent infinite recursion
         }
 
         $visited[$oid] = $entity; // mark visited