XPath queries match nodes in the default namespace

When querying an XML document that explicitly defines the default XML
namespace the XPath evaluator now correctly matches all nodes within
that namespace if no namespace prefix is given in the query. Previously
this would always return an empty set.

lib/oga/xpath/evaluator.rb

@@ -1657,6 +1657,11 @@ module Oga
           ns_matches = true
         end
 
+        if !ns and !ns_matches
+          ns_matches = xml_node.respond_to?(:default_namespace?) &&
+            xml_node.default_namespace?
+        end
+
         return name_matches && ns_matches
       end