Add missing property to expected fixture

Author: Philipp Alferov

test/fixtures/expected.fixture.js

@@ -1,11 +1,15 @@
 module.exports = [{
   id: 1,
+  parent_id: null,
   children: [{
     id: 2,
+    parent_id: 1,
     children: [{
-      id: 3
+      id: 3,
+      parent_id: 2
     }]
   }]
 }, {
-  id: 4
+  id: 4,
+  parent_id: null
 }];