http: use Symbol for outgoing headers

PR-URL: https://github.com/nodejs/node/pull/10941
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/_http_server.js

@@ -13,6 +13,7 @@ const continueExpression = common.continueExpression;
 const chunkExpression = common.chunkExpression;
 const httpSocketSetup = common.httpSocketSetup;
 const OutgoingMessage = require('_http_outgoing').OutgoingMessage;
+const outHeadersKey = require('internal/http').outHeadersKey;
 
 const STATUS_CODES = exports.STATUS_CODES = {
   100: 'Continue',
@@ -179,7 +180,7 @@ function writeHead(statusCode, reason, obj) {
   this.statusCode = statusCode;
 
   var headers;
-  if (this._headers) {
+  if (this[outHeadersKey]) {
     // Slow-case: when progressive API and header fields are passed.
     var k;
     if (obj) {
@@ -196,7 +197,7 @@ function writeHead(statusCode, reason, obj) {
       }
     }
     // only progressive api is used
-    headers = this._headers;
+    headers = this[outHeadersKey];
   } else {
     // only writeHead() called
     headers = obj;