#1930 assert that a instance of the Mongo query builder is used

The base `update(..)` method and the MongoDB `update(..)` differ in the amount of arguments.
Adding a assertion to make sure the correct one is used.

src/Eloquent/Builder.php

@@ -8,10 +8,12 @@
 use Illuminate\Database\Eloquent\Builder as EloquentBuilder;
 use MongoDB\Driver\Cursor;
 use MongoDB\Laravel\Helpers\QueriesRelationships;
+use MongoDB\Laravel\Query\Builder as MongoDBQueryBuilder;
 use MongoDB\Model\BSONDocument;
 
 use function array_key_exists;
 use function array_merge;
+use function assert;
 use function collect;
 use function is_array;
 use function iterator_to_array;
@@ -71,7 +73,10 @@ public function update(array $values, array $options = [])
             return 1;
         }
 
-        return $this->toBase()->update($this->addUpdatedAtColumn($values), $options);
+        $builder = $this->toBase();
+        assert($builder instanceof MongoDBQueryBuilder);
+
+        return $builder->update($this->addUpdatedAtColumn($values), $options);
     }
 
     /** @inheritdoc */