Ensure timestamps are not generated when explicitly set to false [#5440 state:resolved]

activerecord/lib/active_record/timestamp.rb

@@ -61,7 +61,7 @@ def update(*args) #:nodoc:
     end
 
     def should_record_timestamps?
-      record_timestamps && !partial_updates? || changed?
+      record_timestamps && (!partial_updates? || changed?)
     end
 
     def timestamp_attributes_for_update_in_model