Make sure updated_at dates change when relations do
Include modified date header

Also means this will respond to conditional gets via app
which will be faster than via nginx
Use consistent date time formatting in atom feed

Also use Z for timezone offset as it’s easier to read and might save a few chars
Filter index by date

Hidden option so only really going to be much use in the atom feed as most mortals probably can’t type a valid iso8601 date.
Fix spooky action at a distance failure

The touch of `response_set` added to `Certificate` changed the updated_at date.

Solved by sticking creation in a `Timecop.freeze` block
Merge pull request #957 from theodi/feature-recency-filter-feed

Filter atom feed by modified date