Use mapping.fullpath in omniauth callbacks

We need to use `fullpath` to make scoping work for omniauth callbacks.

Previously, the scope would not be applied to omniauth callbacks:
```ruby
scope '/api' do
  devise_for :users, controllers: { omniauth_callbacks: 'users/omniauth_callbacks' }
end
```
```
...
/api/users/sign_out(.:format)          devise/sessions#destroy
/users/auth/:provider(.:format)        users/omniauth_callbacks#passthru {:provider=>/(?!)/}
...
```

With this patch, the routes look as follows:
```
...
/api/users/sign_out(.:format)          devise/sessions#destroy
/api/users/auth/:provider(.:format)        users/omniauth_callbacks#passthru {:provider=>/(?!)/}
...
```