[FIX] point_of_sale: put order back to xml

Was removed at 467ba386 but need it in blackbox for many terms
The alternative is to add all the fields in export_for_printing but this patch,
even if less modular, is way smaller and safer.

addons/point_of_sale/static/src/js/screens.js

@@ -1432,6 +1432,7 @@ var ReceiptScreenWidget = ScreenWidget.extend({
     print_xml: function() {
         var env = {
             widget:  this,
+            order: this.pos.get_order(),
             receipt: this.pos.get_order().export_for_printing(),
             paymentlines: this.pos.get_order().get_paymentlines()
         };