DOMDocument::saveHtml() generates element without name

If `DOMDocument::saveHtml()` is called with an instance of `DOMDocumentFragment` as the argument it generates an invalid element node without a tag name for the fragment node.
### HHVM Version

hhvm-3.9.1 - 3.12.0
### Standalone code, or other way to reproduce the problem

``` php
$document = new DOMDocument();
$fragment = $document->createDocumentFragment();
$fragment->appendChild($document->createElement('em', 'test'));
$fragment->appendChild($document->createTextNode('test'));

echo $document->saveHtml($fragment);
```
### Expected result

``` html
testtest
```
### Actual result

``` html
<>testtest
```