Added spec-file for SignUpService

  `SignUpService` is fully covered through testing `SignUpController`.
  But _guard_ doesn't run the tests if ´sign_up_service.rb´ is changed
  because there was no `spec/services/sign_up_service_spec.rb`.

  The solution:

  An almost empty `sign_up_service_spec.rb´ which just ensures
  that the sign_up_controller_spec is younger than the
  sign_up_service.rb.