Apply node() type tests to the document too.

When running XPath queries such as "self::node()" the result should be a set
containing the document itself. This in turn fixes expressions such as
descendant-or-self::node()/a.

lib/oga/xpath/evaluator.rb

@@ -550,7 +550,9 @@ module Oga
         nodes = XML::NodeSet.new
 
         context.each do |node|
-          nodes << node if node.is_a?(XML::Node)
+          if node.is_a?(XML::Node) or node.is_a?(XML::Document)
+            nodes << node
+          end
         end
 
         return nodes