Fix posibility to select default value on checkout

Problem: function woocommerce_form_field( $key, $args, $value = null ) is used to generate billing and shipping form with three arguments in templates/checkout/form-billing.php and templates/checkout/form-shipping.php. So $value variable is never equals NULL and it's impossible to change with add_filter value of element with key "default" in array $args.
[sorry for my english]