fix bug call function isLoggedIn in facebookAgent return wrong result

[thuydx55]

I used this code in my project

ctor: function() {
    this._agent = window.facebook || (window["plugin"] ? window["plugin"]["FacebookAgent"]["getInstance"]() : null);
},

isLoggedIn: function() {
    return this._agent.isLoggedIn();
},

when i call isLoggedIn function, it always returns false even if i logged in before

[thuydx55]

@minggo @pandamicro pls check this