fix missing cleanup, if response socket is no longer writeable (issue 1547) (#1593)

Relates to issue #1547, see my [comment
there](#1547 (comment))

Co-authored-by: Jan Kaiser <[email protected]>
Co-authored-by: titanism <[email protected]>

Author: 3 people

lib/application.js

@@ -258,9 +258,10 @@ 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