Avoid helper call with default parameters

lib/api/merge_requests.rb

@@ -59,7 +59,7 @@ module API
           else merge_requests
           end
-        merge_requests = merge_requests.reorder(issuable_order_by => issuable_sort)
+        merge_requests = merge_requests.reorder(params[:order_by] => params[:sort])
         present paginate(merge_requests), with: Entities::MergeRequest, current_user: current_user, project: user_project
       end