[IMP] website_sale : avoid duplicated order lines

Add line id to while calling _cart_update() from sale_get_order()
The missing line_id parametre was making the _cart_find_product_line() call to fail as it was linked to an option while searching for lines without options (making the method recreate new lines).
Fixes #2573
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64