use auto_url_for to get url of resource

resource_path get the current resource path (doesn't use the parameter provided)

So for instance, on `/collections/1`, we have collection memberships. And we want to sort collection memberships.

`resource_path` will return the collection path and the collection membership path.

In Rails worls, we would use `url_for`, ... but in ActiveAdmin world, we use `auto_url_for` for that.

So basically, this allows to use the activeadmin-sortable gem in admin show pages.

```ruby
  show do |c|
    attributes_table do
      row :id
      row :name
    end

    panel 'Contents' do
      table_for c.collection_memberships do
        sortable_handle_column
        column :position
        column :collectable
      end
    end
  end
end
```

Note that the sortable action should be on the other model, here CollectionMemberships for instance.