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