Ticket #8614: Correct default page cache url

Currently if you use page cache, Rails will take the default route for
controller the controller to construct the path.

If you wanted to cache the root url (see example below), Rails will create
the cache at /welcome/index.html. Not really what we want.

map.root :controller => "welcome" # Page cache defaults to
"public/welcome/index.html"

Defaulting to "request.path" makes everything a lot simpler, especially when
dealing with caching different formats.

http://dev.rubyonrails.org/ticket/8614