Cache expiry helper methods in 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.

[this branch](https://github.com/alphagov/contacts-frontend/compare/respect-content-store-cache-expiry-headers) uses the `expires_in` helper.

2 corrections made in tests:
1. comment changed to 7 minutes, as it should be
2. `NetHTTP::Response` headers hash has symbolised keys, so changed stubs to match that.