String as return

If the route return code is an int, sinatra use it as http status code

lib/extreme_startup/web_server.rb

@@ -133,7 +133,7 @@ def to_json(*a)
     end
 
     post '/advance_round' do
-      question_factory.advance_round
+      question_factory.advance_round.to_s
     end
 
     post '/pause' do