XPath queries using the default XML namespace do not appear to work (any more)

From !187 (comment 250179630):

Here:

require "oga"

document = Oga.parse_xml('<root xmlns="http://example.com"><bar>bar</bar></root>')

document.xpath('xmlns:root/xmlns:bar')
# => NodeSet()

document.xpath('xmlns:root[namespace-uri() = "http://example.com"]/xmlns:bar')
# => NodeSet()

Each xpath() call returns empty node set but should return node set with an element.

These queries do work in Nokogiri.