Fix broken search results after GDS API Adapters change

The new version of GDS API Adapters changes the type of return values
from API calls. It now returns a GdsApi::Response instead of native
types like Array or Hash. GdsApi::Response quacks like a Hash but it
isn't one.

This switch on the concrete class is a code smell, and changing to
switch on respond_to? is not much of an improvement. I think that this
switch may no longer be needed as we'll always get a hash back, but need
to investigate.

I've added a tech debt story to improve the rummager integration
tests:

https://www.pivotaltracker.com/story/show/60324804
Merge pull request #1020 from alphagov/fix-broken-filters

Fix broken search results after GDS API Adapters change
Avoid test interference

since we are not using transactional fixtures, we need to
ensure that the database is cleaned up after the tests run.
Merge commit 'bbae180eb89db6dc2eb14ebb7180173ffd8a2862' into HEAD