Missing basepath in callback redirections in Nancy

Nancy supports subdirectory structures like `http://some_domain:1234/some_sub_dir/authentication/redirect/some_provider`. But while using this in SimpleAuthentication I noticed that the callback redirections were linked to a location without subdirectories, like: `http://some_domain:1234/authentication/authenticatecallback`.

This small change should fix the problem: it prepends the callback route with the basepath of the URL. If there's no basepath, there won't be any prefixes as basepath will be null and therefore substituted with an empty string.