Fix undefined variable error in controller helper

[k-p-jones]

Within lib/devise/api/controllers/helpers.rb the resource_class is being passed into Devise::Api::Responses::ErrorResponse on initialization. resource_class is a method inherited from the devise controllers so unfortunately, if we use authenticate_devise_api_token! in any controller that doesn't inherit from one, the gem will throw for blank, expired and revoked tokens, rather than handling it cleanly and returning the proper error.

I've added what I think is a clean fix for the issue, and some accompanying specs. I'm happy to make any tweaks if needed :)

This issue was initially raised here #24 (comment)