[4.2] Fix PDO::FETCH_ASSOC Support In Schema::hasColumn()

Consider this a continuation of #3512. When fetch mode is set to `PDO::FETCH_ASSOC`, `Schema::hasColumn()` fails because the processors under `Illuminate\Database\Query\Processors` expect an array of objects. This PR casts results to objects before trying to use them. I've also added tests for each changed processor to ensure that both `PDO::FETCH_ASSOC` and `PDO::FETCH_CLASS` are supported.