Set response header to a valid date in the past.

Change the response header from 'Expires=0' to 'Expires=Fri, 01 Jan 1990 00:00:00 GMT'

From: https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching_FAQ

"It is interesting to note that "Expires: 0" and "Pragma: no-cache" are technically invalid response headers. If none of these headers are present, then the expiration calculation is essentially the algorithm described in RFC 2616 section 13.2."
Merge pull request #1 from adamrights/master

Set response header to a valid date in the past.