Only render json for unhandled exceptions

REFERENCES:

https://www.pivotaltracker.com/story/show/83067926
Looks like you're creating a route which always fails just so you can call it in a test. You should probably use [Anonymous controller](https://relishapp.com/rspec/rspec-rails/v/3-1/docs/controller-specs/anonymous-controller).

It might looks something like this:

```ruby
context "when an exception is raised" do
  controller do
    def index
      fail
    end
  end

  it 'returns json' do
    get :index
    # expectations here
  end
end
```

Then there's no need to add a route or action to `ApplicationController`.