Add 'last-modified'/'if-modified-since' date-based caching.

[lxe]

The current ETag caching works well for the public registry; however, some nginx and other load-balancer and proxy implementations might have different behavior, and only respond with a 304 if an if-modified-since header is sent with the request.

The header shoud be created the same way the if-none-match header is currently created, but instead of the cached etag header, it should send the value of the last-modified header.

I believe most browsers currently operate in this fashion as well.

What do you think?