rack: second attempt to fix flakey test

Author: kratob

rack/test/registering_handler/rack/handler/registering_myself.rb

@@ -1,8 +1,10 @@
+require 'rack/handler'
+
 module Rack
   module Handler
     class RegisteringMyself
     end
 
     register :registering_myself, RegisteringMyself
   end
-end
+end