Allow page cache location to be set to a Pathname

[indirect]

Now that Rails.root is a Pathname, it seems like you should be able to do this:

config.action_controller.page_cache_directory = Rails.root.join("public/cache")

This patch (and test) allow you to do that. Without it, you get inexplicable and impenetrable 500 errors with logs that (super helpfully) look like this:

Started GET "/" for 127.0.0.1 at Sat Feb 05 15:19:06 -0800 2011
  Processing by MediaController#index as XML
Rendered media/index.xml.builder (0.2ms)
Write page /index.html (0.4ms)
Completed   in 6ms

Errno::ENOENT (No such file or directory - ):

[spastorino]

Pushed, thanks.