Add ignore param for Model::assign

```php
$_POST = array(
    'type' => 'mechanical', 
    'name' => 'Astro Boy',
    'year' => 1952, 
    'token' => '123456'
);
$robot = new Robots();
$robot->assign($_POST, array('type' => 1, 'name' => 1, 'year' => 1), TRUE);
```