ignore errors checking existence of relations

I have a column named `guard`, and the code `$this->model->$relation()` executes Eloquent::guard() method to check whether relation method exists, and the exception occurs; `Argument 1 passed to Illuminate\Database\Eloquent\Model::guard() must be of the type array, none given`. 

Another column name, the same name of Eloquent methods exists, will have the same problem, so I think it's good to change ignoring errors like `@$this->model->$relation()`.

Thanks.
Merge pull request #56 from ishinao/patch-1

ignore errors checking existence of relations