XMLDecl mixed in xpath `descendant::node()` match result

[tompng]

XMLDecl should be excluded

# Expected
Nokogiri::XML.parse('<?xml version="1.0"?><root></root>').xpath("descendant::node()")
# => [#<Nokogiri::XML::Element:0xb38 name="root">]

# Actual
REXML::XPath.match REXML::Document.new('<?xml version="1.0"?><root></root>'), "descendant::node()"
# => [<?xml ... ?>, <root/>]
REXML::VERSION
# => "3.4.4"

[tompng]

After #315, optimizable descendant doesn't have this problem any more.

REXML::XPath.match REXML::Document.new('<?xml version="1.0"?><root></root>'), "descendant::node()"
# => [<root/>]

But still not fixed for unoptimizable descendant xpath

REXML::XPath.match REXML::Document.new('<?xml version="1.0"?><root></root>'), "descendant::node()[position()*0!=42]"
# => [<?xml ... ?>, <root/>]