:last css selector not working in DocumentFragment

[brain-geek]

Test to reproduce bug:

  def test_last_fragment
    text = '<p>Lorem ipsum</p><p>Second paragraph</p><div>Not paragraph</div>'
    fragment = Nokogiri::HTML.fragment text

    assert_equal(1, fragment.search('p:last').size)
    assert_equal(1, fragment.css('p:last').size)
  end

Looks like this works with document, but fails in fragment.

[flavorjones]

This is a dup of either #213 or #370.

[flavorjones]

Folding this into #572, the underlying issue is the same.