saml: use get_or_create() in save_key_values (fixes #6883)

With a simple Model.save() the second save in case of replay fails
because the created is updated to the NULL value. It seem that
initialization of DateTime field is ignored when Django detects that a
save is an UPDATE and not an INSERT.

src/authentic2/saml/models.py

@@ -819,7 +819,11 @@ class KeyValue(models.Model):
         verbose_name_plural = _("key value associations")
 
 def save_key_values(key, *values):
-    KeyValue(key = key, value = values).save()
+    # never update an existing key, key are nonces
+    kv, created = KeyValue.objects.get_or_create(key=key, defaults={'value': values})
+    if not created:
+        kv.value = values
+        kv.save()
 
 def get_and_delete_key_values(key):
     try: