[4.2] Make "findClosure" Actually Find Closures

Return of method findClosure() is Closure Object that is callable and must be an object. So only using is_callable() function isn't enough. A string could be callable, example when you adding alias 'app' in your route. 
Route::get('apps', array('as' => 'app', function()
{
//
})); will make an error.

![screenshot from 2014-12-03 23 16 49](https://cloud.githubusercontent.com/assets/7432391/5283824/8d8de5f6-7b42-11e4-9d91-df07599cc8e9.png)