Allow subscribers to be set as authors in WordPress

functions.php

<?php

add_filter( 'wp_dropdown_users_args', 'add_subscribers_to_dropdown', 10, 2 );
/**
 * Allow subscribers to be set as authors
 *
 * @link http://wordpress.stackexchange.com/a/216117
 *
 * @since 1.0.0
 *
 * @param $query_args
 * @param $r
 *
 * @return mixed
 */
function add_subscribers_to_dropdown( $query_args, $r ) {
	$query_args['who'] = '';

	return $query_args;
}