Return nil for expires if the Expires header is not parseable.

We encountered a site which had an Expires header which was unparseable by `Time.httpdate`. This caused `Time.httpdate` to raise [`ArgumentError: not RFC 2616 compliant date`](https://github.com/ruby/ruby/blob/v2_0_0_481/lib/time.rb#L509).

This PR makes `expires` return `nil` if the Expires header is not parseable.