Fix SplDoublyLinkedList when using array syntax

Fixes #1415

If you append items to a `SplDoublyLinkedList` using array syntax (`$foo[] = 'bar'`), the tail would not be updated.

**Example:**

```
<?php
$stack = new SplStack();

$stack[] = "var1";
$stack[] = "var2";
$stack[] = "var3";

foreach ($stack as $var) {
    echo $var . "\n";
}
```

**HHVM Output:**

```
var1
```

**Correct Output:**

```
var3
var2
var1
```