lib: change anonymous function to arrow function

PR-URL: #24589
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>

lib/_http_client.js

@@ -299,7 +299,7 @@ ClientRequest.prototype.abort = function abort() {
   if (this.res) {
     this.res._dump();
   } else {
-    this.once('response', function(res) {
+    this.once('response', (res) => {
       res._dump();
     });
   }