optimize XML serialization

[ThomasSevestre]

This PR improves performance of XML serialization :

• by 15% without active support
• by 25% when active support is loaded

This has a visible impact in our application using ActiveRecord. I have a 15% overall gain.

I'm using this script to benchmark :

# require "active_support/all"
require "benchmark"
module Xlsxtream
  n = 500000
  Benchmark.bm do |x|
    x.report { for i in 1..n; Row.new(['foo', nil, 23, 'foo', nil, 23, 'foo', nil, 23, 'foo', nil, 23], 1).to_xml; end }
  end
end