Add option to use request.format to determine caches_page extension

caches_page will treat automatically assume url with a period (.) on it to be a file extension. This can cause unexpected behavior with caching pages with a period in it (i.e. www.example.com/users/show/user....

Here's a patch and tests that would allow the option, :use_request_extension, to use format.request to determine the extension and it would not append to the cached_page if it already has the extension (i.e. www.example.com/users/user.name.....