Use namespace_name in Attribute#to_xml

Instead of using `namespace.name` lets just use `namespace_name`. This fixes the
problem of serializing attributes where the namespace prefix is "xmlns" as the
namespace for this isn't registered by default.

This fixes #47.

lib/oga/xml/attribute.rb

@@ -82,7 +82,7 @@ module Oga
       #
       def to_xml
         if namespace_name
-          full_name = "#{namespace.name}:#{name}"
+          full_name = "#{namespace_name}:#{name}"
         else
           full_name = name
         end