/search response is now always hash

Please DO NOT MERGE until https://github.com/alphagov/govuk_content_api/commit/d18c39697c90a41ad20be82a6a23d15b643a309b has been deployed, at which point all clients are compatible.

Hash is much more flexible. This change is safe to make once all clients are requesting/expecting a hash.

Turns out there were quite a few tests using the previous default behaviour.