module custom form alter needs adding class but zurb remove it

zurb-foundation/inc/form.inc: $form['actions']['submit']['#attributes'] = array('class' => array('secondary', 'button', 'radius'));

It's cleanning all previous classes, in my module i'm trying:

$form['actions']['submit']['#attributes']['class'][] = 'some-class';

without effect

I know i can change my module weight but i don't think zurb theme must clean previous added classes

Meanwhile i'll copy form.inc alter code to my template.php.