[#72069960] Add test to ensure unpublished data-sets are not cached

- Sets cache control to 'no-cache' based on reading through things like http://stackoverflow.com/questions/1046966/whats-the-difference-between-cache-control-max-age-0-and-no-cache?answertab=votes#tab-top
[#72069960] Add in cache control switch in query responses

- `fetch` Could possibly do with being refactored so that we use our cache_control class methods to serve the fetch response at all times, rather than playing around with the response headers in the method. Probably alright for the context of [this story](https://www.pivotaltracker.com/story/show/72069960) though
remove unnecessary call to revalidate
Merge pull request #308 from alphagov/cache_aff

Set cache control to 'no-cache' for unpublished data-sets [#72069960]