chore: Reduce unnecessary variable declarations (#1298)

Author: zhaosaisai

lib/application.js

@@ -200,9 +200,9 @@ function respond(ctx) {
   // allow bypassing koa
   if (false === ctx.respond) return;
 
-  const res = ctx.res;
   if (!ctx.writable) return;
 
+  const res = ctx.res;
   let body = ctx.body;
   const code = ctx.status;