[FIX] event: keep default values when registering from frontend

Purpose
=======

When creating an attendee from frontend some fields may be added in
parameters, notably through inheritance (questions, ...). We want the
defaults values to have the priority on those parameters (e.g. barcode code).

Task ID-2322411
PR odoo/odoo#

addons/event/models/event.py

@@ -450,7 +450,11 @@ class EventRegistration(models.Model):
             'partner_id': partner_id.id,
             'event_id': event_id and event_id.id or False,
         }
-        data.update({key: value for key, value in registration.items() if key in self._fields})
+        data.update({
+            key: value for key, value in registration.items()
+            if key in self._fields and key not in data and not self._fields[key].default
+        })
+
         return data
 
     def do_draft(self):