Avoid assert_nil deprecation warning in role_presenter_test
Avoid assert_nil deprecation warning in role_presenter_test

The `current_person` method does return a
`RolePresenter::UnassignedPersonPresenter`, but that object responds to
`#nil?` with `true` because it delegates that method to the first
argument passed to its constructor (`nil`). Using `assert x==y` avoids
the deprecation warning.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall