add magic __isset method to make laravel object_get helper happy

The Formbuilder calls object_get() helper on a model to populate form fields with default values.

https://github.com/laravel/framework/blob/master/src/Illuminate/Html/FormBuilder.php#L905

 isset() is called on the model attribute although the presenter doesn´t support it.

https://github.com/laravel/framework/blob/master/src/Illuminate/Support/helpers.php#L732