Fix bug when setting a date field with undefined value

lib/instance.js

@@ -298,7 +298,7 @@ module.exports = (function() {
             }
 
             // Convert date fields to real date objects
-            if (this.Model._hasDateAttributes && this.Model._isDateAttribute(key) && value !== null && !(value instanceof Date) && !value._isSequelizeMethod) {
+            if (this.Model._hasDateAttributes && this.Model._isDateAttribute(key) && value !== null && value !== undefined && !(value instanceof Date) && !value._isSequelizeMethod) {
               value = new Date(value);
             }
           }