Added toString method to allow user to be logged in Tomcat

When using the NegotiateSecurityFilter, the logged in user cannot be logged in Tomcat using the (Extended)AccessLogValve. There are a couple of objects in the session that could be logged (but not remote user, it is only available in the wrapped request), if they exposed a toString method. This adds a toString method to WindowsPrincipal, so that the authenticated user can be logged with x-S(waffle.servlet.NegotiateSecurityFilter.PRINCIPAL) using the ExtendedAccessLogValve or %{waffle.servlet.NegotiateSecurityFilter.PRINCIPAL}s using the AccessLogValve.