Nokogiri `add_child` does not copy namespaces

add_child_ns_nokogiri_bug.rb

#!/usr/bin/env ruby

# see https://github.com/sparklemotion/nokogiri/issues/1200

require 'nokogiri'

src =<<EOD
<wrapper xmlns="ns">
    <record xml:id="r1" xmlns:extra="extra">
        <field>aaa</field>
        <field extra:type="second">bbb</field>
    </record>
</wrapper>
EOD

src_doc = Nokogiri::XML(src)
record = src_doc.at('//base:field[. = "bbb"]', {'base' => "ns"})

puts "== Original document"
puts src_doc.to_xml

# if we instead add `record.dup` here, things will work properly
src_doc.root.add_child(record)

puts
puts "== New document"
puts "== There should be a xmlns declaration in the last field element but it is not"
puts src_doc.to_xml