verify Fixed rack's json test

rack/config.ru

@@ -11,7 +11,7 @@ app = lambda do |env|
     [
       200,
       { 'Content-Type' => 'application/json' },
-      [{:message => "Hello World!"}.to_json]
+      [{:message => "Hello, World!"}.to_json]
     ]
   end
 end