SEBA-301 Only save changed state

Change-Id: I177e2d6eee878551cc2236deabfddc920a553868
diff --git a/xos/synchronizer/event_steps/auth_event.py b/xos/synchronizer/event_steps/auth_event.py
index 6786c98..e534aee 100644
--- a/xos/synchronizer/event_steps/auth_event.py
+++ b/xos/synchronizer/event_steps/auth_event.py
@@ -38,4 +38,4 @@
             raise Exception("authentication.events: Cannot find att-workflow-driver service instance for this event")
 
         si.authentication_state = value["authenticationState"];
-        si.save(update_fields=["authentication_state", "updated"], always_update_timestamp=True)
+        si.save_changed_fields(always_update_timestamp=True)