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.
Adds Rummager test helpers module

Adds stubbing helper for Rummager unified search used to retrieve
services and information data for an org.

Story: https://www.pivotaltracker.com/story/show/75026206
Merge pull request #173 from alphagov/add-rummager-test-helpers

Adds Rummager test helpers module

https://www.pivotaltracker.com/story/show/75026206
Version bump to 12.1.0

* Add Rummager test helpers
Remove to-do list from the README.

The README isn't the place for it, and it's not detailed enough to be helpful.
Merge pull request #174 from alphagov/remove-to-do

Remove to-do list from the README
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
Release 12.2.0

Add .expires_in, .expires_at to GDSApi::Response
Updates services and info data fixture

* Adds previously missing "link" key

* Services and info data requests to Rummager are now returned ordered by
title

* Changes example query to include ordering by title
Adds no services and info data found helper
Add draft functionality to `content_api.tags`.

This param is available for all content_api endpoints
but is currently not used except on #tags.
Merge pull request #175 from alphagov/add-draft-to-content-api-tags-method

Add draft functionality to `content_api.tags`.
Version 12.3.0
Merge pull request #177 from alphagov/bump-version-for-draft-tags

Version 12.3.0
Merge pull request #176 from alphagov/update-rummager-test-helpers

Update rummager test helpers

https://www.pivotaltracker.com/story/show/75342126
Version 12.4.0

Adds `rummager_has_no_services_and_info_data_for_organisation` to
Rummager test helpers
Merge pull request #178 from alphagov/version-12.4.0

Version 12.4.0
Fixes services and info data fixture