Problem creating an array instance in a widget in 4.4 update

Question

Before WordPress 4.4 recent update, I was able to create an array instance in a widget from a multiple select menu this way:

<?php $cats = get_categories(); ?>  
<select multiple="multiple" name="<?php echo $this->get_field_name('cats][') ?>">
    <?php foreach ( $cats as $cat ) { ?>
        <option value="<?php echo $cat->term_id ?>"><?php echo $cat->name; ?></option>
    <?php } ?>
</select>

The type of $instance['cats'] generated from this code was an array before that update, now it's a string.

What has changed and how can I solve it?

=======

UPDATE: Complete code of a widget:

###################################################################

function widet_sample() {
    register_widget('widet_sample');
}
add_action('widgets_init', 'widet_sample');

class widet_sample extends WP_Widget {

    function widet_sample() {
        parent::__construct(false, 'SAMPLE');
    }

    function update($new_instance, $old_instance) {
        $instance = $old_instance;
        $instance['cats'] = $new_instance['cats'];
        return $instance;
    }

    function form($instance) {
?>
<p>
    Categories IDs: <?php print_r($instance['cats']); ?>
</p>
<p>
    <?php $cats = get_categories(); ?>  
    <select class="widefat" multiple="multiple" name="<?php echo $this->get_field_name('cats][') ?>">
        <?php foreach ( $cats as $cat ) { ?>
            <option value="<?php echo $cat->term_id ?>"><?php echo $cat->name; ?></option>
        <?php } ?>
    </select>
</p>    
<?php
    }
}

###################################################################

Answer 1

Problem solved! Still don't know what has changed in the 4.4 update though.

Prior to 4.4, you can use the square brackets reverted to generate an array instance, like that:

<select multiple="multiple" name="<?php echo $this->get_field_name('cats][') ?>">
//$instance['cats'] = array();

Starting from 4.4, you can use square brackets in regular case, like that:

<select multiple="multiple" name="<?php echo $this->get_field_name('cats[]') ?>">
//$instance['cats'] = array();

Or, use the generic and best approach:

<select multiple="multiple" name="<?php echo $this->get_field_name('cats') ?>[]">
//$instance['cats'] = array();