middleware that preserves flashes over multiple redirects

flash_preserver.rb

# use via this in application.rb:
#    config.middleware.insert_after(ActionDispatch::Flash, FlashPreserver)

class FlashPreserver
  def initialize(app)
    @app = app
  end

  def call(env)
    flash = (session = env['rack.session']) && session['flash']

    @app.call(env).tap do |status,*rest|
      flash.keep if flash && [301, 302, 307].include?(status)
    end
  end
end