How to test session attribute set in Interceptor

[Niels Jørgen Bagger]

I have a simple Interceptor that intercepts all requests:

package proxy.interceptors

import grails.util.Holders
import proxy.Principal


class ProxyInterceptor {

   int order = HIGHEST_PRECEDENCE

   ProxyInterceptor() {
      matchAll()
   }

   boolean before() {
      Principal principal = new Principal(Holders.config, request)
      request.session.principal = principal

      return true
   }

}

How do a write a test, that tests that principal is actually set on the session?