[FIX] website_sale: fix order for the pager in front end.

In psql, use LIMIT and OFFSET together without a fully specified and uniq sort order
will generate unexpected behaviour.

Eg:
> id   id_dept  name
> -------------------
> 1    1        Tom
> 2    1        Mike
> 3    2        Meggie
> 4    2        Marge
> 5    3        Bart
> 6    3        Lisa

> using LIMITed selects like:

>    SELECT * FROM employee ORDER BY id_dept LIMIT 3
>    SELECT * FROM employee ORDER BY id_dept LIMIT 3 OFFSET 3
>    SELECT * FROM employee ORDER BY id_dept LIMIT 3 OFFSET 6

> You can have some result missings from the 3 requests, and others duplicated.
> Because id_dept is not a uniq order.

addons/website_sale/controllers/main.py

@@ -106,7 +106,8 @@ class WebsiteSale(http.Controller):
 
     def _get_search_order(self, post):
         # OrderBy will be parsed in orm and so no direct sql injection
-        return 'website_published desc,%s' % post.get('order', 'website_sequence desc')
+        # id is added to be sure that order is a unique sort key
+        return 'website_published desc,%s , id desc' % post.get('order', 'website_sequence desc')
 
     @http.route(['/shop/change_pricelist/<model("product.pricelist"):pl_id>'], type='http', auth="public", website=True)
     def pricelist_change(self, pl_id, **post):