Ability to set max-age in content item test stub

also, adding date as HTTP/1.1 specs say origins
servers should add Date header.
Add .expires_in, .expires_at to GDSApi::Response

www.agileplannerapp.com/boards/173808/cards/4800

these methods expose the expiration time in seconds
and absolute time value respectively, by inferring
them from max-age or expires values received in
response from content store.
Guarding against missing Date header
Merge pull request #172 from alphagov/new-cache-expiry-helper-methods

Cache expiry helper methods in GDSApi::Response