This PR introduces a test to avoid regression on a functionality used to implement lazy loading in doctrine/common#168.

The concept is quite simple:

class A 
{
    public $b;
    public function __get($name)
    {
        echo 'called';
    }
}

$a = new A();
unset ($a->b);
$a->b; // produces output 'called'

This functionality initially looked like a bug to me, but it turned out to be expected behaviour after asking around for quite a bit. The PR introduces a test to protect this functionality.