Implement Wayback Machine's X-Archive-Orig-last-modified HTTP header
We just got this request from archive.org on bug-wget@gnu.org mailing list.
Here is an example URL: https://web.archive.org/web/20030613111637/http://www.gnu.org:80
X-Archive-Orig-last-modified
replaces Last-Modified
(which isn't there anyways) to provide the document's original time stamp.