Disabled serializing of Singletons

Disabled serializing of Singletons

$a = Test::getInstance();
$b = Test::getInstance();
echo 'A: '.$a->foo."\n"; // A: bar
echo 'B: '.$b->foo."\n"; // B: bar
$a->foo = 'a';
echo 'A: '.$a->foo."\n"; // A: a
echo 'B: '.$b->foo."\n"; // B: a
$b = unserialize(serialize($b)); // -> With this change SystemException
$a->foo = 'b';
echo 'A: '.$a->foo."\n"; // A: b
echo 'B: '.$b->foo."\n"; // B: a