test: expand test-http-response-multiheaders test

Include writeHead in the test per @bnoordhuis' suggestion

test/parallel/test-http-response-multiheaders.js

@@ -15,19 +15,40 @@ const norepeat = [
 ];
 
 const server = http.createServer(function(req, res) {
-  for (let name of norepeat) {
-    res.setHeader(name, ['A', 'B']);
+  var num = req.headers['x-num'];
+  if (num == 1) {
+    for (let name of norepeat) {
+      res.setHeader(name, ['A', 'B']);
+    }
+    res.setHeader('X-A', ['A', 'B']);
+  } else if (num == 2) {
+    let headers = {};
+    for (let name of norepeat) {
+      headers[name] = ['A', 'B'];
+    }
+    headers['X-A'] = ['A', 'B'];
+    res.writeHead(200, headers);
   }
-  res.setHeader('X-A', ['A', 'B']);
   res.end('ok');
 });
 
 server.listen(common.PORT, common.mustCall(function() {
-  http.get({port:common.PORT}, common.mustCall(function(res) {
-    server.close();
-    for (let name of norepeat) {
-      assert.equal(res.headers[name], 'A');
-    }
-    assert.equal(res.headers['x-a'], 'A, B');
-  }));
+  for (let n = 1; n <= 2 ; n++) {
+    // this runs twice, the first time, the server will use
+    // setHeader, the second time it uses writeHead. The
+    // result on the client side should be the same in
+    // either case -- only the first instance of the header
+    // value should be reported for the header fields listed
+    // in the norepeat array.
+    http.get(
+      {port:common.PORT, headers:{'x-num': n}},
+      common.mustCall(function(res) {
+        if (n == 2) server.close();
+        for (let name of norepeat) {
+          assert.equal(res.headers[name], 'A');
+        }
+        assert.equal(res.headers['x-a'], 'A, B');
+      })
+    );
+  }
 }));