kirki: Reset doesn't work for Background field.

Hi Ari.

I found nothing in the documentation for this field. How do I set the default image?

Kirki::add_field( 'ramsay_customize', array(
    'type' => 'background',
    'settings' => 'footer_background',
    'label' => esc_html__( 'Footer Background', 'ramsay' ),
    'description' => esc_html__( 'Select background for footer', 'ramsay' ),
    'section' => 'section_footer_styling',
    'priority' => $priority++,
    'default' => array(
        'background-color' => '#252525',
        'background-image' => $theme_path_images . 'footer-bg.png',
        'background-size' => 'cover'
    ),
    'output' => array(
        array(
            'element' => '.vl-footer-outer',
        ),
    )
) );