Don't set Response::expires() in Response::cache()

According to the rfc2616,

If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header, even if the Expires header is more restrictive.

So, it's pretty much useless to set Response::expires().
http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html