Use Zendesk's fix for delete with symbol keys in rails 2

Ticket : https://envato.lighthouseapp.com/projects/5394-marketplace/tickets/6051-sessiondelete-doesnt-work-with-symbol-keys-in-rails-2#ticket-6051-2

Paired with @lukearndt 

This implements Zendesk's fix for [this problem](https://github.com/envato/rails_4_session_flash_backport/issues/3).

We have been unable to run the Rails 2 spec folder locally. It throws an exception when it attempts to require 'action_controller'. Any help from someone who has worked with Rails 2 would be appreciated.

Note that the specs don't run on master either; same issue.