[PATCH] Bug 422: RFC 2616 Date header requirements

AFAICT the Date: header is already implemented on all generated pages
and ICAP processed pages.
This tests and enforces Date: on all outgoing replies as required.

I'm not certain this is the right place, it appears to be post-caching.
The RFC indicates the Date: should be enforced pre-caching. But was
unable to find a place of input cloning/processing after initial parse.

If anyone can point it out to me I will be grateful. There are other
processing needing it to be added there too.

Amos