Builder options on Ruby Nokogiri :standalone

I would like to to create XML that begins with:

<?xml version = "1.0" encoding = "UTF-8" standalone ="no"?>

But I cannot find how to add the 'standalone' option in the Nokogiri documentation.

My code is like this:

builder = Nokogiri::XML::Builder.new(:encoding => 'UTF-8', :standalone => 'no') do |xml|

But it fails when Nokogiri finds :standalone. The :encoding works.