Fix FileStore cleanup method

The cleanup method on ActiveSupport::Cache::FileStore on the 3.0 branch is defined but doesn't actually work. I have fixed the implementation so it can be used to clean up old cache entries either by expiration time or by how recently they've been used and added tests.

This method is needed to keep the cache from growing without bound until if fills up the disk.

The syntax is:

Rails.cache.cleanup(:expired_only => true)

or

Rails.cache.cleanup(:not_accessed_in => 2.days)