fix bug: error happens if no callback is set when calling client.close()

[laike9m]

Hi mcavage, I've encountered a bug when invoking client.close()

/Users/laike9m/ICT/ICT_Projects/zk/node_modules/zkplus/node_modules/once/once.js:16
    return f.value = fn.apply(this, arguments)
                        ^
TypeError: Cannot call method 'apply' of undefined
    at f (/Users/laike9m/ICT/ICT_Projects/zk/node_modules/zkplus/node_modules/once/once.js:16:25)
    at Client.<anonymous> (/Users/laike9m/ICT/ICT_Projects/zk/node_modules/zkplus/lib/client.js:215:17)
    at Client.f (/Users/laike9m/ICT/ICT_Projects/zk/node_modules/zkplus/node_modules/once/once.js:16:25)
    at Client.g (events.js:180:16)
    at Client.emit (events.js:117:20)
    at Client.defaultStateListener (/Users/laike9m/ICT/ICT_Projects/zk/node_modules/zkplus/node_modules/node-zookeeper-client/index.js:52:14)
    at Client.emit (events.js:95:17)
    at Client.onConnectionManagerState (/Users/laike9m/ICT/ICT_Projects/zk/node_modules/zkplus/node_modules/node-zookeeper-client/index.js:262:14)
    at ConnectionManager.emit (events.js:95:17)
    at ConnectionManager.setState (/Users/laike9m/ICT/ICT_Projects/zk/node_modules/zkplus/node_modules/node-zookeeper-client/lib/ConnectionManager.js:168:14)

The reason is clear: In lib/client.js line 198

cb = once(cb);

and this is what once do

function once (fn) {
  var f = function () {
    if (f.called) return f.value
    f.called = true
    return f.value = fn.apply(this, arguments)
  }
  f.called = false
  return f
}

Therefore, even though cb is undefined, it becomes a function after calling once(cb), so in line 209 and line 214, the if(cb) judgement breaks cause it's always true. Then cb gets called, but since fn is undefiend, fn.apply(this, arguments) raise an error.

I've made the fix so that if cb is undefined, once(cb) won't be called, and the subsequent judgement won't break.