Description:
------------
If class implements \Serializable interface and serializes same instance of \DateTime object - deserialization fails with Notice:  Notice: unserialize(): Error at offset 13 of 157 bytes in /opt/project/testme.php on line 12

Test script:
---------------
class A implements \Serializable
{
    public function serialize()
    {
        $dt = new \DateTime();
        return serialize(array($dt, serialize([$dt])));
    }

    public function unserialize($serialized)
    {
        $a = unserialize($serialized);
        unserialize($a[1]);
    }
}

$a = new A();
$s = serialize($a);
$d = unserialize($s);

Expected result:
----------------
Empty screen, no notices and proper de-serialized value

Actual result:
--------------
Notice: unserialize(): Error at offset 13 of 157 bytes in /opt/project/testme.php on line 12
And properly deserialized \DateTime object