Description:
------------
When trying to store class property values in $_SESSION the whole class is put into session.

Reproduce code:
---------------
for($i=0; $i <= $this->numItems;$i++){
    $_SESSION['cart']['numItems'] = $this->numItems;
    $_SESSION['cart']['items'][$i]['isbn'] = $this->items[$i]['isbn'];
    $_SESSION['cart']['items'][$i]['quantity'] = $this->items[$i]['quantity'];
}

Expected result:
----------------
Array
(
    [numItems] => 1
    [items] => Array
        (
            [1] => Array
                (
                    [isbn] => BR15609
                    [quantity] => 1
                )
        )
)

Actual result:
--------------
Array
(
    [cart] => shoppingcart Object
        (
            [numItems] => 1
            [items] => Array
                (
                    [1] => Array
                        (
                            [isbn] => BR15609
                            [quantity] => 1
                        )

                )
        )
)