dgram: changes Socket.close() to return itself

This commit adds a return statement to the dgram.Socket.close()
function that returns itself after it finishes. This follows along
the functionality of the more popular and, dare I say, father-library
`lib/net.js`.

lib/dgram.js

@@ -358,6 +358,8 @@ Socket.prototype.close = function() {
   this._handle.close();
   this._handle = null;
   this.emit('close');
+
+  return this;
 };
 
 

test/parallel/test-dgram-close.js

@@ -32,7 +32,7 @@ buf.fill(42);
 var socket = dgram.createSocket('udp4');
 var handle = socket._handle;
 socket.send(buf, 0, buf.length, common.PORT, 'localhost');
-socket.close();
+assert.strictEqual(socket.close(), socket);
 socket = null;
 
 // Verify that accessing handle after closure doesn't throw