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

spec/oga/xpath/evaluator/axes/self_spec.rb

@@ -74,5 +74,17 @@ describe Oga::XPath::Evaluator do
         @set[0].should == @document.children[0].children[1]
       end
     end
+
+    context 'matching the document itself' do
+      before do
+        @set = @evaluator.evaluate('self::node()')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the document itself' do
+        @set[0].is_a?(Oga::XML::Document).should == true
+      end
+    end
   end
 end