header-exposer.rb

require 'rack'

app = Proc.new do |env|
  if env['PATH_INFO'] == '/301'
    next ['301', {'Content-Type' => 'text/html', 'X-Auth-Token' => 'This-Should-Be-Forwarded', 'Location' => '/headers'}, []]
  end

  if env['PATH_INFO'] == '/302'
    next ['302', {'Content-Type' => 'text/html', 'X-Auth-Token' => 'This-Should-Be-Forwarded', 'Location' => '/headers'}, []]
  end

  if env['PATH_INFO'] == '/headers'
    headers = env.select {|k,v| k.start_with? 'HTTP_'}
    .collect {|pair| [pair[0].sub(/^HTTP_/, ''), pair[1]]}
    .collect {|pair| pair.join(": ") << "<br>"}
    next ['200', {'Content-Type' => 'text/html'}, headers]
  end

  body = """
  <form method=post action='/301'>
    <input type=submit value=301>
  </form>

  <form method=post action='/302'>
    <input type=submit value=302>
  </form>
  """
  ['200', {'Content-Type' => 'text/html'}, [body]]
end

Rack::Handler::WEBrick.run app