handle manually set update ts

so the create and update are having the same logic of 3 conditions: 
 a. key given;
 b. new model / value changed; 
 c. ts not manually set;

lib/base/model.js

@@ -643,7 +643,7 @@ ModelBase.prototype.timestamp = function(options) {
   const updatedAtKey = timestampKeys[1];
   const isNewModel = method === 'insert';
 
-  if (updatedAtKey && (isNewModel || this.hasChanged())) {
+  if (updatedAtKey && (isNewModel || this.hasChanged()) && !this.hasChanged(updatedAtKey) ) {
     attributes[updatedAtKey] = now;
   }