Refactor application.js - use arrow function in callback

lib/application.js

@@ -115,9 +115,7 @@ module.exports = class Application extends Emitter {
       res.statusCode = 404;
       const ctx = this.createContext(req, res);
       onFinished(res, ctx.onerror);
-      fn(ctx).then(function() {
-        respond(ctx);
-      }).catch(ctx.onerror);
+      fn(ctx).then(() => respond(ctx)).catch(ctx.onerror);
     };
   }