fix issue with multiple fields identifier

Form/DataTransformer/ModelToIdTransformer.php

@@ -56,6 +56,6 @@ class ModelToIdTransformer implements DataTransformerInterface
             return null;
         }
 
-        return current($this->modelManager->getIdentifierValues($entity));
+        return $this->modelManager->getNormalizedIdentifier($entity);
     }
 }

Tests/Form/DataTransformer/ModelToIdTransformerTest.php

@@ -71,8 +71,8 @@ class ModelToIdTransformerTest extends \PHPUnit_Framework_TestCase
     public function testTransform()
     {
         $this->modelManager->expects($this->once())
-            ->method('getIdentifierValues')
-            ->will($this->returnValue(array(123)));
+            ->method('getNormalizedIdentifier')
+            ->will($this->returnValue(123));
 
         $transformer = new ModelToIdTransformer($this->modelManager, 'TEST');