Fix wrapInner()

Hi!!

I'd like to suggest this patch.
This PR fixes the following behavior of `wrapInner()`:

Test script:
```php
$qp = qp('<?xml version="1.0"?><root><div>Text A</div><div>Text B</div></root>');
$qp->find('div')->wrapInner('<test/>')->writeXML();
```

Expected result:
```xml
<?xml version="1.0"?><root><div><test>Text A</test></div><div><test>Text B</test></div></root>
```

Actual result:
```xml
<?xml version="1.0"?><root><div><test>Text AText B</test></div><div/></root>
```