http: reuse existing StorageObject

PR-URL: nodejs#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_outgoing.js

@@ -20,10 +20,6 @@ var RE_FIELDS = new RegExp('^(?:Connection|Transfer-Encoding|Content-Length|' +
 var RE_CONN_VALUES = /(?:^|\W)close|upgrade(?:$|\W)/ig;
 var RE_TE_CHUNKED = common.chunkExpression;
 
-// Used to store headers returned by getHeaders()
-function OutgoingHeaders() {}
-OutgoingHeaders.prototype = Object.create(null);
-
 var dateCache;
 function utcDate() {
   if (!dateCache) {
@@ -510,7 +506,7 @@ OutgoingMessage.prototype.getHeaderNames = function getHeaderNames() {
 // Returns a shallow copy of the current outgoing headers.
 OutgoingMessage.prototype.getHeaders = function getHeaders() {
   const headers = this[outHeadersKey];
-  const ret = new OutgoingHeaders();
+  const ret = new StorageObject();
   if (headers) {
     const keys = Object.keys(headers);
     for (var i = 0; i < keys.length; ++i) {