[Bug] Config::has() does not work properly

[franzliedke]

For a non-existing config file named 'key.php', Config::has('key') returns true.

That's because Input::get('key') returns an empty array, which is different than the default value generated in the has() method.

The logic of that method could be improved to really check for the existence of such an element.

[taylorotwell]

Added Config::hasGroup to address this.