When grouping a collection by a date field an exception is thrown

[JakeBooher]

• Laravel Version: 7.22.4
• PHP Version: 7.3.20
• Database Driver & Version: MySQL 8.0

Description:

When using group on a collection, and setting the key to a datetime casted field you get an exception:

ErrorException
array_key_exists(): The first argument should be either a string or an integer

Coming from the following:

            foreach ($groupKeys as $groupKey) {
                $groupKey = is_bool($groupKey) ? (int) $groupKey : $groupKey;

                if (! array_key_exists($groupKey, $results)) {
                    $results[$groupKey] = new static;
                }

                $results[$groupKey]->offsetSet($preserveKeys ? $key : null, $value);
            }

Steps To Reproduce:

1. Create a timestamp field type (in my case, I'm using date)
2. Set your model to cast this field to datetime
3. Group by this field

        $events = Event::where([
            ['starts_at', '>=', $rangeStart],
            ['ends_at', '<=', $rangeEnd],
        ])->orderBy('starts_at', 'desc')->get();

        $events = $events->groupBy('date');