cluster.disconnect callback getting called too early in 0.11

``` JavaScript
var cluster = require('cluster');

if (cluster.isMaster) {
  // Fork workers.
  cluster.fork();
  cluster.fork();

  // "called when all workers are disconnected [...]"
  cluster.disconnect(function(){
    for(var prop in cluster.workers){
      var worker = cluster.workers[prop];
      console.log(worker.state);
    }
  });

} else {
  require('net').createServer().listen(8000);
}
```
It seems like the disconnect callback is getting called to early. If I loop through the workers I still get one with the 'online' state. This does not happen on 0.10

Expected output: nothing or disconnect/exit
got: online

node version: v0.11.7
OS: Ubuntu Linux 64bit

**Edit:**

Sorry for the pull request hassle. I'm not so familiar with the way github handles them.