I found a bug with a model with default_scope and add a test

My project use default_scope for multi-tenancy and my `User` model is nested, associate with self. 

```ruby
class User < ActiveRecord::Base
  default_scope { Registry.current_user.self_and_descendants if Registry.current_user.present? }
end
```

The problem is: if i create a user with default_scope situation it will store wrong `#lft` & `#rgt`

If this is a bug, i can fix it.