Set expiration time based on GDSApi::Response

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

Caching should be controllable from the origin of
the content. This will allow us to do things like
scheduled publishing better.
Merge pull request #11 from alphagov/respect-content-store-cache-expiry-headers

Set expiration time based on GDSApi::Response