SplFixedArray can not be serialized

After serialize-deserialize any object of SplFixedArray (perhaps any SplQueue, SplStack etc.), empty object in result.

Example:

``` php

$serializer = new JsonSerializer();

$obj = new SplFixedArray(2);
$obj[0] = 1;
$obj[1] = 'test';

$serialized = $serializer->serialize($obj); // {"@type": "SplFixedArray"}
$serializer->unserialize($serialized); // SplFixedArray Object ()

```
