use flat concatenation instead of nesting

Author: Nikolai-Kircher

lib/cancan/model_adapters/active_record_adapter.rb

@@ -202,7 +202,7 @@ def merge_non_empty_conditions(behavior, conditions_hash, sql)
         when false_sql
           behavior ? conditions : false_sql
         else
-          behavior ? "(#{conditions}) OR (#{sql})" : "not (#{conditions}) AND (#{sql})"
+          behavior ? "(#{conditions}) OR #{sql}" : "(not (#{conditions}) AND #{sql})"
         end
       end