Zend\Form openTag() method not including all attributes

<?php
$form->setAttribute('action', $this->url('customer', array('action' => 'index')));
$form->setAttribute('ng-submit', 'validateForm()');
$form->prepare();
var_dump($form->getAttributes());
echo $this->form()->openTag($form);
?>

Well, as you can see I'm setting some form attributes ans the dumping them with var_dump($form->getAttributes()); and here is the result :

array (size=4)
    'method' => string 'get' (length=3)
    'name' => string 'searchCustomerForm' (length=18)
    'action' => string '/customer' (length=9)
    'ng-submit' => string 'validateForm()' (length=4)

but the result form tag is :

<form id="searchCustomerForm" action="/customer" name="searchCustomerForm" method="get" class="ng-pristine ng-valid">  

the ng-submit attribute is missing!!

How Can I set form attributes?