[Bug] Order ID filter incorrect

Hi,

Just a quick little bug report (+ Fix!!)

In marketpress.php, inside the function generate_order_id() is the following code:

$_SESSION['mp_order'] = apply_filters( 'mp_order_id', $order_id ); //Very important to make sure order numbers are unique and not sequential if filtering
    return $order_id;

If you filter the order ID, then the value set in the session is different from the value returned, the code should read:

$_SESSION['mp_order'] = apply_filters( 'mp_order_id', $order_id ); //Very important to make sure order numbers are unique and not sequential if filtering
    return $_SESSION['mp_order'];