Allow other roles in Author metabox

<?php

function filter_wp_dropdown_users_args($query_args) {
	$query_args['who'] = ''; // reset the query
    $query_args['capability'] = ''; // reset the query
	$query_args['role__in'] = array( 'subscriber', 'administrator' );
    return $query_args;
}
add_filter('wp_dropdown_users_args', 'filter_wp_dropdown_users_args',100,1);