fix(http): use the statusCode from the writeHead too

Author: gergelyke

lib/instrumentations/core/http/server.js

@@ -91,11 +91,13 @@ function wrapListener (listener, agent, mustCollectStore) {
 
     response.once('finish', instrumentedFinish)
 
-    response.writeHead = function () {
+    response.writeHead = function (statusCode) {
+      var _statusCode = statusCode || response.statusCode
+
       serverSendTime = microtime.now()
 
       // collected because previous reason like (x-must-collect etc.) or wrong status code
-      if (response.statusCode > 399) {
+      if (_statusCode > 399) {
         mustCollectStore[requestId] = consts.MUST_COLLECT.ERROR
       }