Default namespace declaration with empty string as prefix should not break attribute selection

[martin-honnen]

The fix suggested some months ago to allow using an empty string as the prefix to select e.g. HTML element in the XHTML namespace unfortunately breaks attribute selection, i.e. a test like

      [Fact]
        public void BindingEmptyPrefixShouldNotBreakAttributeSelection()
        {
            var namespaceManager = new XmlNamespaceManager(new NameTable());
            namespaceManager.AddNamespace("", "http://www.w3.org/1999/xhtml");

            var nodeList = GetXHTMLSampleDoc().XPath2SelectNodes("//@lang", namespaceManager);

            Assert.Equal(1, nodeList.Count);
        }

fails as the XPath2 code now looks for attributes named lang in XHTML namespace. I am currently not sure how to fix the NameTest/QName production in XPath.y to only use the context.NamespaceManager.DefaultNamespace if the node name is of an element node.