Increase code coverage.

Author: drupol

spec/drupol/phptree/Converter/GraphSpec.php

@@ -22,7 +22,7 @@ public function it_can_generate_a_graph()
         $level2 = new Node();
 
         $root
-            ->add($level1, $level2);
+            ->add($level1, $level2, $level2);
 
         $this
             ->convert($root)