Return element names when no namespace

When the XML is not in a namespace, all the element names are set to null.

This can be remedied by replacing https://github.com/fruux/sabre-xml/blob/master/lib/Sabre/XML/Reader.php#L56 with:

```
return $this->localName;
```

Not sure if this causes any issues elsewhere. But I know the testGetClarkNoNS will need to be updated to pass.
