Let’s talk about pretty tedious task - upgrading rails, of course it can be interesting experience, but if you have relatively large application to upgrade it can be cumbersome, here are my thoughts after doing through process of switching Rails 3.2 to Rails 4.1.

config.action_dispatch.default_headers = {
  # documentation specifies ALLOWALL, but empty string
  # should work across all common browsers
  'X-Frame-Options' => ''
}

The list goes on, but every app is different, so I will stop here and wish you best luck with your upgrade process ;-). Let the 200 response status be with you, always.