[FIX] pos: do not mix create and validation date

The validation_date was using the creation_date value in the follwing scenario:
1. create a new order with a few lines (creation_date=t1)
2. reload the page, `init_from_JSON` uses validation_date=t1
3. validate the order at t2, ignored due to already stored value

creation_date must be kept for tracablity reasons (e.g. pos_blackbox_be)

Fixes https://www.odoo.com/groups/59/20542134

addons/point_of_sale/static/src/js/models.js

@@ -1782,7 +1782,7 @@ exports.Order = Backbone.Model.extend({
     },
 
     initialize_validation_date: function () {
-        this.validation_date = this.validation_date || new Date();
+        this.validation_date = new Date();
     },
 
     set_tip: function(tip) {