[5.8] Change visibility to public for hasPivotColumn method by pactode

[pactode]

I was trying to do a check of whether a relation contains a specific pivot column. This is currently not possible. In order to fix this I suggest changing the visibility to public.

Example of usage:

$relation = $model->somePivotRelation();

if ($relation->hasPivotColumn('position')) {
    // build a specific query containing that attribute
}

Let me know what you think about this.