fix: added missing cleanup, if response socket is no longer writeable

Author: Jan Kaiser

lib/application.js

@@ -221,9 +221,10 @@ function respond (ctx) {
   // allow bypassing koa
   if (ctx.respond === false) return
 
-  if (!ctx.writable) return
-
   const res = ctx.res
+
+  if (!ctx.writable) return res.end()
+
   let body = ctx.body
   const code = ctx.status