Gather the JS/CSS from the render array, the html head from the band data.
protected function gatherHeaders() {
// Simple replacement for head
if (isset($this->storage['head'])) {
$this->storage['head'] = str_replace($this->storage['head'], '', drupal_add_html_head());
}
else {
$this->storage['head'] = '';
}
$attached = drupal_render_collect_attached($this->storage['output']);
$this->storage['css'] = $attached['css'];
$this->storage['js'] = $attached['js'];
}