Fixing issue #107

what if the base url is ""? and should the pos not be `=== 0`?

how does this work with multidomain situations? in https://github.com/FriendsOfSymfony/FOSHttpCacheBundle/pull/148 we noticed that you may invalidate a different domain than the one set in the base url... though i really don't know how this would be handled with the different location pattern for nginx. but at least the same location should not break.