no-cache issue Safari and Mozilla Firefox

Hi Folks,

I'm writing a multistep form with the the help of Ryan Bates

http://railscasts.com/episodes/217-multistep-forms

but I want change a little the form after the confirmation redirect to
the notification page that appear to the user to communicate that the
form just filled out I want that the user can't back with the 'Browser
Back Button' at the confirmation page (the previous page)

to explain better what I want for me:

A = is the confirmation page

B = is the notification page

Thanks to Google I found this page

http://blog.serendeputy.com/posts/how-to-prevent-browsers-from-caching-a-page-in-rails/

And to the application_controller.rb I put

  before_filter :set_cache_buster

  def set_cache_buster
    response.headers["Cache-Control"] = "no-cache, no-store, max-age=0,
must-revalidate"
    response.headers["Pragma"] = "no-cache"
    response.headers["Expires"] = "Fri, 01 Jan 1990 00:00:00 GMT"
  end

so when the user is on B when click on the browser back button on Safari
a dialog box appear

if click on {Send Button} appear an Rails error page

***********************************************
NoMethodError in FormonlineController#create

undefined method `deep_merge!' for nil:NilClass

...

24. session[:formonline_params].deep_merge!(params[:formonline]) if
params[:formonline]

This is telling you that session[:formonline_params] has been nulled
out. Try detecting that condition and, if so, setting it to empty (or
some other sensible value), before this line. That will at least
bypass this crash, though I don't know if it will let the back-button
work as you desire.

-Dave