Fix SQL query when sorting inherited product fields

Some inherited fields are not directly stored in the database, such field use
a custom SQL order. When sorting from product, we must reuse the same custom
SQL order.

issue5318
review30861002

product.py

@@ -164,7 +164,8 @@ class TemplateFunction(fields.Function):
                     }
             else:
                 template = tables['template']
-            return [Column(template, name)]
+            return getattr(Template, name).convert_order(
+                name, tables['template'], Template)
         return order