mark default radio button selected with bootstrap active class

sample:

echo $this->Form->input('start_minutes', array(
	'type' => 'radio',
	'class' => 'btn btn-default',
	'div' => false,
	'legend' => false,
	'hiddenField' => false,
	'default' => '0',
	'options' => array('0' => __('now'), '15' => __('in 15 min.'), '30' => __('in 30 min.'))));