Handle nested arrays when printing cron values

Handle nested arrays in cron values by using array_walk_recursive to output $info['args']

Fixes the following notice:
Notice: Array to string conversion in wp-includes/formatting.php on line 3196

On a dev environment where notices are displayed, this may cause the "Installed Plugins" page to appear blank (at least when xdebug is formatting the notice output).

Updated to use a method instead of anonymous function for PHP 5.2 compatibility.