Description:
------------
SplStack object serialization works wrong

Test script:
---------------
$t = new SplStack();
$t->push(1);
var_dump(unserialize(serialize($t)));

Expected result:
----------------
object(SplStack)#32 (2) { ["flags":"SplDoublyLinkedList":private]=> int(6) ["dllist":"SplDoublyLinkedList":private]=> array(1) { [0]=> int(1) } } 

Actual result:
--------------
object(SplStack)#31 (2) { ["flags":"SplDoublyLinkedList":private]=> int(6) ["dllist":"SplDoublyLinkedList":private]=> array(0) { } }