How do I test a devise parameter sanitizer

I've implemented the code below so I can update user attributes via strong parameters. It's straight out of devise's documentation and I understand it pretty well.

The problem however is that I'm really new to testing. I'm using SimpleCov and it tells me the u.permit bit is not covered. Unfortunately, I'm not quite sure how to test for this.

I'm using rspec and I'd like to know if anyone could point me in the right direction.

How do I test for this?

devise_parameter_sanitizer.for(:account_update) do |u|
  u.permit(:username, :email, :password, :password_confirmation)
end