lib: http server multi same-named headers support

Let http server support response multiple same-named header as RFC2612
defined, this feature is also used in http/2.0 "server push" feature.

Fix nodejs#3591

lib/_http_outgoing.js

@@ -350,7 +350,10 @@ OutgoingMessage.prototype.setHeader = function(name, value) {
  if (this._headers === null)
  this._headers = {};
 
- var key = name.toLowerCase();
+ var key = name.toLowerCase(), serial = 0, origKey = key;
+ while (this._headers[key] !== undefined) {
+ key = (origKey + serial++);
+ }
  this._headers[key] = value;
  this._headerNames[key] = name;
 
@@ -408,7 +411,18 @@ OutgoingMessage.prototype._renderHeaders = function() {
 
  for (var i = 0, l = keys.length; i < l; i++) {
  var key = keys[i];
- headers[headerNames[key]] = headersMap[key];
+ if (headers[headerNames[key]] !== undefined) {
+ var isArray = Array.isArray(headers[headerNames[key]]);
+ if (isArray) {
+ headers[headerNames[key]].push(headersMap[key]);
+ } else {
+ var orig = headers[headerNames[key]];
+ headers[headerNames[key]] = [];
+ headers[headerNames[key]].push(orig);
+ }
+ } else {
+ headers[headerNames[key]] = headersMap[key];
+ }
  }
  return headers;
 };