Add support for models using default_scope

Weird but it actually need:

```ruby
@model_class.unscoped do
  @scope ? @scope.send(resource_name).scope : @model_class.unscoped
end
```

I would then keep the current implementation. Feels right?

P.S. : DO NOT USE `default_scope`! It's clearly a terrible thing with lot of black magic and downsides.