mark default radio button selected with bootstrap active class

[geskill]

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.'))));