Added specific test when X-Forwarded-For is 'unkown'

The new version of Squid sets X-Forwarded-For to 'unkown' by default.
This currently is a problem in Rails 3, fortunately not in Rack.
Added this test for regression purposes only.
Also see http://www.squid-cache.org/Doc/config/forwarded_for/

As requested by Konstantin :)