[FIX] website_sale: Don't overwrite address behavior on partner

When the ecommerce module is installed, the partner field's context is overwritten, which causes some features to be lost, e.g. showing the customer address.

This commit fixes the above by merging the content of both contexts.

closes odoo/odoo#42554

X-original-commit: a7027a18
Signed-off-by: Luis González [Vauxoo] <luisg123v@users.noreply.github.com>