Allow 0 as a valid Expires header

This is the most convenient way to indicate "already expired" as it requires no date calculation and is immune to timezone differences.

From the spec:

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.21

> HTTP/1.1 clients and caches MUST treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired")