threadsafety bug (in ActionController::Caching::Sweeper)

https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/3457-actioncontrollercachingsweeper-controller-instance-is-not-thread-safe

I ran in to this error "NoMethodError (undefined method
`controller_name' for nil:NilClass)" when trying to use
config.threadsafety!. The patch by the OP works for me. I'm on Rails
2.3.10, but glancing at the code I believe it applies to Rails 3 as
well.

-Greg