Added XML::Document#root_node

lib/oga/xml/document.rb

@@ -54,6 +54,18 @@ module Oga
         end
       end
 
+      ##
+      # Returns self.
+      #
+      # This method exists to make this class compatible with Element, which in
+      # turn makes it easier to use both in the XPath compiler.
+      #
+      # @return [Oga::XML::Document]
+      #
+      def root_node
+        self
+      end
+
       ##
       # Converts the document and its child nodes to XML.
       #