fixes rails/activerecord-session_store rails#172

Author: endangurura

lib/action_dispatch/session/active_record_store.rb

@@ -119,7 +119,7 @@ def get_session_model(request, id)
         logger.silence do
           model = get_session_with_fallback(id)
           unless model
-            id = generate_sid
+            id ||= generate_sid # id = generate_sid
             model = @@session_class.new(:session_id => id.private_id, :data => {})
             model.save
           end