Merge pull request #361 from gahia/fix_raw_sql_sort_order

Fix raw SQL argument deprecation warning

Author: seuros

lib/closure_tree/support_attributes.rb

@@ -80,7 +80,7 @@ def dont_order_roots
     end
 
     def nulls_last_order_by
-      "-#{quoted_order_column} #{order_by_order(true)}"
+      Arel.sql "-#{quoted_order_column} #{order_by_order(true)}"
     end
 
     def order_by_order(reverse = false)