DOMQuery::text() does not work as intended

Hi!

I'm not sure whether this is a bug or I am doing something wrong.

When using the text() method to set text contents of all matched elements,
I get the following result:

Test script:
```php
<?php
$xml = '<?xml version="1.0"?><root><div>Text A</div><div>Text B</div></root>';
qp($xml, 'div')->text('New Text')->writeXML();
```

Expected result:
```xml
<?xml version="1.0"?><root><div>New Text</div><div>New Text</div></root>
```

Actual result:
```xml
<?xml version="1.0"?><root><div /><div>New Text</div></root>
```

This Pull Request fixes this behavior and adds an unit test for it.