Returning root element instead of document node from JsonToXml()

As discussed in the issue [U4-3581](http://issues.umbraco.org/issue/U4-3581) it's better to return the actual element instead of the document node.

I am not able to run the tests, but as far as I can see, they should pass because of the way the C# objects handle this - otherwise we'll have to get someone like @leekelleher to have a quick look at them :-)