Make dependency on action_controller explicit

The `responders.rb` file requires the whole of `action_controller`, not just `action_controller/base` (which can't be loaded on its own). Requiring only `action_controller/base` causes dependency errors in projects that don't explicitly require `action_controller` elsewhere.

Once `action_controller` is required in `responders.rb` it can be removed from `test_helper.rb`.