Fix NodeContainer::addChildAt method

`NodeContainer::addChildAt` method works correctly only if `$index` argument is equal to `0`.

Here is an example of incorrect behavior:
```php
$fragment = html('fragment');
$fragment->addChild('0');
$fragment->addChild('1');
$fragment->addChild('2');
$fragment->addChildAt('+', 1);

echo $fragment;
```
outputs `012+` instead of `0+12`.

The PR fixes the problem.