Response/XmlVisitor cannot accept 'xmlns' in response.

The `XmlVisitor` class cannot deal with responses where the root node (or any other node for that matter) has an `xmlns` declaration. This is very common when interacting with SOAP services, RSS responses, etc.

```php
<?php

$xml = <<<XML
<foo xmlns="urn:foo"><bar:Bar xmlns:bar="urn:bar">test</bar:Bar></foo>
XML;

$simple = simplexml_load_string($xml);

print_r(json_decode(json_encode($simple), true));
```

The above does not output the expected result. The code is identical to the one used in the `XmlVisitor::before` method.