http: avoid duplicate keys in writeHead

Use setHeader in writeHead to avoid sending duplicate headers

Fixes #5036

lib/_http_server.js

@@ -187,9 +187,6 @@ ServerResponse.prototype.writeHead = function(statusCode) {
   var obj = arguments[headerIndex];
 
   if (obj && this._headers) {
-    // Slow-case: when progressive API and header fields are passed.
-    headers = this._renderHeaders();
-
     if (util.isArray(obj)) {
       // handle array case
       // TODO: remove when array is no longer accepted
@@ -207,8 +204,10 @@ ServerResponse.prototype.writeHead = function(statusCode) {
       var keys = Object.keys(obj);
       for (var i = 0; i < keys.length; i++) {
         var k = keys[i];
-        if (k) headers[k] = obj[k];
+        if (k) this.setHeader(k, obj[k]);
       }
+      // Slow-case: when progressive API and header fields are passed.
+      headers = this._renderHeaders();
     }
   } else if (this._headers) {
     // only progressive api is used

test/simple/test-http-write-head.js

@@ -0,0 +1,46 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+var http = require('http');
+
+// Verify that ServerResponse.writeHead() works as setHeader.
+// Issue 5036 on github.
+
+var s = http.createServer(function(req, res) {
+  res.setHeader('test', '1');
+  res.writeHead(200, { Test: '2' });
+  res.end();
+});
+
+s.listen(common.PORT, runTest);
+
+function runTest() {
+  http.get({ port: common.PORT }, function(response) {
+    response.on('end', function() {
+      assert.equal(response.headers['test'], '2');
+      assert(response.rawHeaders.indexOf('Test') !== -1);
+      s.close();
+    });
+    response.resume();
+  });
+}