Don't merge value objects

The bundle currently tries to call $om->merge() on value objects,
defined with (local), too. This causes:

Notice: Undefined offset: 0 in vendor/doctrine/orm/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php line 729

This PR checks whether an object has identity before trying to merge it.

Alice/Loader.php

@@ -71,7 +71,13 @@ public function setObjectManager(ObjectManager $manager)
 
         $newReferences = array();
         foreach ($this->references as $name => $reference) {
-            $newReferences[$name] = $this->persister->merge($reference);
+            // Don't merge value objects, e.g. Doctrine embeddables
+            $metadata = $manager->getClassMetadata(get_class($reference));
+            if (count($metadata->getIdentifier()) > 0) {
+                $reference = $this->persister->merge($reference);
+            }
+
+            $newReferences[$name] = $reference;
         }
         $this->references = new ArrayCollection($newReferences);