Add remote_photo_url and make SamlIdp::Zendesk::Attributes#xml return a Nodeset

We do not want to return an entire xml document from the attributes
because these get plugged into the middle of the response xml document.
This changes the attributes object to return a nodeset. It also adds the
remote_photo_url handling. Lastly, it adds the implementation for the
to_s tests that essentially acts as an acceptance test for the class.