Add method getting non deleted record after unscoped

A proposal to solve problem that `unscoped` removing a check of deleted or not.
I'd unscope only default_scope by just a method.

```ruby
class Model < AR::Base
  acts_as_paranoid
  default_scope ->{ :updated_at > 3.days.ago }
end

Model.all #=> 'where deleted_at = NULL and updated_at > 3.days.ago'
Model.unscoped #=> 'nothing condition'
Model.active_on_unscoped #=> 'where deleted_at = NULL'
```