prevent unexpected file deletion from CDN

I finally found the reason for the sudden removal of files from the CDN.
There are "X-Delete-At/After" headers in the CloudFiles, which does the next thing:
`Objects that are assigned the X-Delete-At or X-Delete-After header are deleted within one day of the expiration time, and the object stops being served immediately after the expiration time.`
(http://docs.rackspace.com/files/api/v1/cf-devguide/content/Expiring_Objects-e1e3228.html)