dgram: change Socket.bind() to return itself

This commit changes `lib/dgram.js` Sockets to, when
they are bound to a port / IP, return themselves. This
is done in order to allow chaining of methods and be
in accordance with the `lib/net.js` library.

lib/dgram.js

@@ -170,7 +170,7 @@ Socket.prototype.bind = function(port /*, address, callback*/) {
   if (port instanceof UDP) {
     replaceHandle(self, port);
     startListening(self);
-    return;
+    return self;
   }
 
   var address;
@@ -231,6 +231,8 @@ Socket.prototype.bind = function(port /*, address, callback*/) {
       startListening(self);
     }
   });
+
+  return self;
 };
 
 

test/parallel/test-dgram-bind.js

@@ -29,4 +29,6 @@ socket.on('listening', function () {
   socket.close();
 });
 
-socket.bind(); // should not throw
+var result = socket.bind(); // should not throw
+
+assert.strictEqual(result, socket); // should have returned itself