Fix tests.

Author: drupol

spec/drupol/phptree/Modifier/RemoveNullNodeSpec.php

@@ -4,6 +4,7 @@
 
 namespace spec\drupol\phptree\Modifier;
 
+use drupol\phptree\Exporter\Ascii;
 use drupol\phptree\Modifier\RemoveNullNode;
 use drupol\phptree\Node\ValueNode;
 use PhpSpec\ObjectBehavior;