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

Author: PeterDaveHello

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;
 };