Bug 422: RFC 2616 Date header requirements

Bug 422: RFC 2616 Date header requirements

The Date: header appears to already be implemented on all generated
pages and ICAP processed pages.
This tests and enforces Date: on all other 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.
The storeEntry timestamp is used to estimate correct receiving date.