dgram: fix no address bind()

I broke dgram.Socket#bind(port, cb) almost a year ago in 332fea5a but
it wasn't until today that someone complained and none of the tests
caught it because they all either specify the address or omit the
callback.

Anyway, now it works again and does what you expect: it binds the
socket to the "any" address ("0.0.0.0" for IPv4 and "::" for IPv6.)

lib/dgram.js

@@ -176,6 +176,7 @@ Socket.prototype.bind = function(/*port, address, callback*/) {
 
   var port = arguments[0];
   var address = arguments[1];
+  if (typeof address === 'function') address = '';  // a.k.a. "any address"
 
   // resolve address first
   self._handle.lookup(address, function(err, ip) {

test/simple/test-dgram-bind-default-address.js

@@ -0,0 +1,36 @@
+// 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 dgram = require('dgram');
+
+dgram.createSocket('udp4').bind(common.PORT + 0, common.mustCall(function() {
+  assert.equal(this.address().port, common.PORT + 0);
+  assert.equal(this.address().address, '0.0.0.0');
+  this.close();
+}));
+
+dgram.createSocket('udp6').bind(common.PORT + 1, common.mustCall(function() {
+  assert.equal(this.address().port, common.PORT + 1);
+  assert.equal(this.address().address, '::');
+  this.close();
+}));