Description:
------------
ArrayObject descendants can be forced to serialize public/protected/private properies using __sleep(), but it produces E_NOTICE



Reproduce code:
---------------
class Test extends ArrayObject
{
	public $var = 123;

	public function __sleep()
	{
		return array('var');
	}
}

$test = new Test;
$s = serialize($test);


Expected result:
----------------
none

Actual result:
--------------
Notice: serialize() [function.serialize]: "var" returned as member variable from __sleep() but does not exist