lingo: make sure session is saved after lingo_next_url is set (#22933)

combo/apps/lingo/views.py

@@ -372,6 +372,7 @@ class PayView(View):
             # it if payment is canceled
             request.session.setdefault('lingo_next_url',
                                 {})[transaction.order_id] = request.build_absolute_uri(next_url)
+            request.session.modified = True
 
         # XXX: mark basket items as being processed (?)