Add support for a random date in the past

I'm using rack-nocache for security reasons in an application, but I wasn't too keen on having the "Fri, 29 Aug 1997 02:14:00 EST" fingerprint sent with all of my headers, since that might leak information about what server tools we are using.

This commit generates a random RFC 1123 date occurring between 25 years ago and one month ago, and uses it instead for the "Expires" header.