Fix by adding deeper loop to unserialise meta values

After fix serialized array in meta value is properly expanded:

```json
{
    "post_meta": {
        "has_array_of_array": [
            [
                0,
                1,
                2,
                3,
                4,
                5
            ]
        ]
    }
}
```

Now using `&$variable` reference instead of array indexes (should be more
memory efficient and is easier to write).