(performance) improved optClone checks

lib/model.js

@@ -311,7 +311,7 @@ var optClone = Model.prototype.__optClone = Model.prototype.$optClone = function
   options = options || {};
   return Utils.cloneDeep(options, function(elem) {
     // The InstanceFactories used for include are pass by ref, so don't clone them.
-    if (elem &&
+    if (elem && typeof elem === 'object' &&
       (
         elem._isSequelizeMethod ||
         elem instanceof Model ||