[FIX] website_sale: sale ribbon not saving

Current behaviour:
When editing a product, try to add a ribbon,
it will not save.

Steps to reproduce:
1. Go to website
2. Go to Shop
3. Click on Edit (top right)
4. Select any product
5. At Badge: Select Sale
6. (badge is visible)
7. Click on Save
8. (badge is gone)

Cause of the issue:
The element was not considered "dirty"

opw-3487397

closes odoo/odoo#135008

Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>