$config should be an array because of \Arr::merge

If the original model does not have a config file but the extended one does, 
the \Arr::merge would raise an error without the default "array()" value.
Merge pull request #13 from vrcAlbert/master/0.1.5

$config should be an array because of \Arr::merge
Fixed PR 13
Merge master/0.1.5