Add test for Birthday cast

Author: yguedidi

tests/GraphNode/GraphNodeTest.php

@@ -22,6 +22,7 @@
  */
 namespace Facebook\Tests\GraphNode;
 
+use Facebook\GraphNode\Birthday;
 use Facebook\GraphNode\GraphNode;
 use PHPUnit\Framework\TestCase;
 
@@ -106,6 +107,13 @@ public static function provideInvalidDateTimeFieldValues()
         yield ['foo_time', 'Expected the invalid ISO 8601 format to fail.'];
     }
 
+    public function testCastBirthdayFieldValueToBirthday()
+    {
+        $graphNode = new GraphNode(['birthday' => '11/02/1989']);
+
+        $this->assertInstanceOf(Birthday::class, $graphNode->getField('birthday'));
+    }
+
     public function testGettingGraphNodeAsAnArrayWillNotUncastTheDateTimeObject()
     {
         $graphNode = new GraphNode([