Correct handling namespaced resources

[yivo]

Fixes issue #348.
Recipe to reproduce bug:
Add route:

    mount_devise_token_auth_for 'Project::ACL::User', at: 'api/auth'

Sign in and then try to validate token.
You will get:

Processing by DeviseTokenAuth::TokenValidationsController#validate_token as */*
Completed 500 Internal Server Error in 3ms (ActiveRecord: 0.0ms)

NoMethodError (undefined method `name' for nil:NilClass):
  devise (3.5.1) app/controllers/devise_controller.rb:38:in `resource_name'
  devise_token_auth (0.1.34) app/controllers/devise_token_auth/application_controller.rb:10:in `resource_class'
  devise_token_auth (0.1.34) app/controllers/devise_token_auth/concerns/set_user_by_token.rb:19:in `set_user_by_token'

[booleanbetrayal]

Thanks for this @yivo