Fix bad tests in spec/app/api_spec.rb

The tests were checking, among other things, that a GET request to `/api/tags` and `/api/collections/policies` returned JSON content, which is a valid expectation. The problem is that both returned JSON content, though `/api/tags` returned a 404 error and `/api/collections/policies` returned a 200. This fixes the bad request to `/api/tags` and adds an additional status code check to make sure that the test will fail if the endpoint doesn't exist.