Monkey patch ActionCachePath to fix a bug

When infer_extension is not set ActionCachePath does not set the
extension from any explicitly specified format, and hence expiry
produces cache keys which don't have any extension and don't match
the keys generated when the cache entry was created.

config/initializers/action_cache_path.rb

@@ -0,0 +1,19 @@
+module ActionController
+  module Caching
+    module Actions
+      class ActionCachePath
+        def initialize(controller, options = {}, infer_extension = true)
+          if infer_extension
+            @extension = controller.params[:format]
+            options.reverse_merge!(:format => @extension) if options.is_a?(Hash)
+          else
+            @extension = options[:format]
+          end
+
+          path = controller.url_for(options).split(%r{://}).last
+          @path = normalize!(path)
+        end
+      end
+    end
+  end
+end