Need to fix a bug of 'disconnection' for 'net' module.

[hyunjune]

`server.on('connection',``` function(socket) {
console.log('Server> a client connected');
socket.on('close', function() {
console.log('Server> a client disconnected');
});
socket.destroy();
});

var socket = new net.Socket();

socket.connect(1234, "127.0.0.1", function(status) {
});

socket.on('close', function() {
console.log('Client> socket closed');
});
`
Above codes, just called 'close' on server-side.
We expected to call both server-side and client-side.