[FIX] website_sale_coupon: update reward line total when changing product qty on cart

- Add a product (imac 1.799$) and a promo code (eg: 10%) in your cart
  - Go to checkout (/shop/cart). You now see 1,619.10 Total and 179.90 discount
  - Change product qty (eg: 2)
  - Subtotal is now 2x 1.799 - 179.90 instead of 2x 1.799 - 2x 179.90
  - Reward line total (discount) is still the same amount (10% on 1 ipad)

Now, we will recompute the discount price based on the entire cart

Note: This behavior was wanted and comes from this commit #87d542
But for the shop, we need to have the reward line being updated on the fly or
it wont make sense for the end user.

This closes #ee4be7a532dd707a0cd19c766dc8e8537dcda242

controllers/main.py

@@ -19,3 +19,9 @@ class WebsiteSale(WebsiteSale):
         order = request.website.sale_get_order()
         order.recompute_coupon_lines()
         return super(WebsiteSale, self).payment(**post)
+
+    @http.route(['/shop/cart'], type='http', auth="public", website=True)
+    def cart(self, **post):
+        order = request.website.sale_get_order()
+        order.recompute_coupon_lines()
+        return super(WebsiteSale, self).cart(**post)

models/__init__.py

@@ -2,4 +2,5 @@
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
 
 import sale_coupon_rule
-import sale_order
+import sale_order
+import website

models/website.py

@@ -0,0 +1,12 @@
+# -*- coding: utf-8 -*-
+from odoo import models, fields, api
+
+
+class Website(models.Model):
+    _inherit = "website"
+
+    @api.multi
+    def sale_get_order(self, force_create=False, code=None, update_pricelist=False, force_pricelist=False):
+        order = super(Website, self).sale_get_order(force_create, code, update_pricelist, force_pricelist)
+        order.recompute_coupon_lines()
+        return order