Failing test for XPath namespace query in VMAP document

:warning: **THIS PULL REQUEST NOT INTENDED FOR MERGE** :warning: 

I'm observing some strange behavior using the Ono library. (To be completely fair, it actually seems to be a problem in libxml2.) This PR isn't so much intended to be merged, as it is to show the strange/buggy behavior.

The problem is that the `-[ONOXMLDocument XPath:]` method fails for the `/vmap:VMAP/vmap:Extensions/uo:unicornOnce` XPath query against the following XML document:

```xml
<vmap:VMAP version="1.0" xmlns:uo="uo" xmlns:vmap="http://www.iab.net/vmap-1.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <vmap:Extensions>
    <uo:unicornOnce />
    <uo:contentImpressions />
    <uo:requestParameters />
  </vmap:Extensions>
</vmap:VMAP>
```

Yet, it works for the following document, which is identical to the above, except in the order in which namespace attribute declarations appear in the root element:

```xml
<vmap:VMAP version="1.0" xmlns:vmap="http://www.iab.net/vmap-1.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:uo="uo">
  <vmap:Extensions>
    <uo:unicornOnce />
    <uo:contentImpressions />
    <uo:requestParameters />
  </vmap:Extensions>
</vmap:VMAP>
```

The root of the problem seems to be that the ONOXMLDocument's `self.xmlNode` doesn't have the `uo` namespace when it calls `xmlXPathRegisterNs()`.

Am I misunderstanding something about XML documents or XML namespaces, or is this a legitimate bug?