Magic method __toString() must return a string

[deminy]

In some PHP IDE (e.g., PhpStorm) you may see that method _\MrRio\ShellWrap::_toString() is highlighted with notification saying

__toString() method must return a string

This could be triggered with following code piece where no actual shell command invoked:

<?php
require_once __DIR__ . '/vendor/autoload.php';

echo (new MrRio\ShellWrap());
?>

Although in reality people may not make method calls like that, it's better to get it fix to make variables consistent. Property \MrRio\ShellWrap::$output should be initialized and used as a string but not an array, as in the fix.