dgram: handle default address case when offset and length are specified

Fixes a regression introduced by: https://github.com/nodejs/node/pull/4374.
Adds a new test to avoid similar issue in the future.
The test is disabled on windows, because this feature never worked
there.

Fixes: https://github.com/nodejs/node/issues/5398
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Evan Lucas <evanlucas@me.com>

lib/dgram.js

@@ -286,6 +286,13 @@ function enqueue(self, toEnqueue) {
 }
 
 
+// valid combinations
+// send(buffer, offset, length, port, address, callback)
+// send(buffer, offset, length, port, address)
+// send(buffer, offset, length, port)
+// send(bufferOrList, port, address, callback)
+// send(bufferOrList, port, address)
+// send(bufferOrList, port)
 Socket.prototype.send = function(buffer,
                                  offset,
                                  length,
@@ -294,8 +301,7 @@ Socket.prototype.send = function(buffer,
                                  callback) {
   var self = this;
 
-  // same as arguments.length === 5 || arguments.length === 6
-  if (address) {
+  if (address || (port && typeof port !== 'function')) {
     buffer = sliceBuffer(buffer, offset, length);
   } else {
     callback = port;

test/parallel/test-dgram-send-default-host.js

@@ -0,0 +1,43 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const dgram = require('dgram');
+
+if (common.isWindows) {
+  // on Windows this test will fail
+  // see https://github.com/nodejs/node/pull/5407
+  console.log('1..0 # Skipped: This test does not apply on Windows.');
+  return;
+}
+
+const client = dgram.createSocket('udp4');
+
+const timer = setTimeout(function() {
+  throw new Error('Timeout');
+}, common.platformTimeout(2000));
+
+const toSend = [new Buffer(256), new Buffer(256), new Buffer(256), 'hello'];
+
+toSend[0].fill('x');
+toSend[1].fill('y');
+toSend[2].fill('z');
+
+client.on('listening', function() {
+  client.send(toSend[0], 0, toSend[0].length, common.PORT);
+  client.send(toSend[1], common.PORT);
+  client.send([toSend[2]], common.PORT);
+  client.send(toSend[3], 0, toSend[3].length, common.PORT);
+});
+
+client.on('message', function(buf, info) {
+  const expected = toSend.shift().toString();
+  assert.ok(buf.toString() === expected, 'message was received correctly');
+
+  if (toSend.length === 0) {
+    client.close();
+    clearTimeout(timer);
+  }
+});
+
+client.bind(common.PORT);