add yet another date format, to support Rack

Although [RFC 2616](http://tools.ietf.org/html/rfc2616#section-3.3.1) requires that "GMT" must be used for the time zone, Rack uses "-0000" instead. The origin of Rack's current behavior is this commit: https://github.com/rack/rack/commit/5e0a9413a80e9b0aa8a74bfc806fb20f2ea0bf3c