Test error fix

Fix for test error.

```
  1) Error:
LevelsControllerTest#test_should_get_edit:
ActionView::Template::Error: undefined method `constantize' for nil:NilClass
    app/views/levels/_form.html.haml:29:in `block in _app_views_levels__form_html_haml___1373745870318909663_70147164958960'
    app/views/levels/_form.html.haml:1:in `_app_views_levels__form_html_haml___1373745870318909663_70147164958960'
    app/views/levels/edit.html.haml:2:in `_app_views_levels_edit_html_haml__3770241133002909610_70147145541320'
    app/controllers/application_controller.rb:66:in `block in with_locale'
    app/controllers/application_controller.rb:65:in `with_locale'
    test/controllers/levels_controller_test.rb:214:in `block in <class:LevelsControllerTest>'
```