issue-41649 collection not possible group by with Stringable

Author: medeiroz

src/Illuminate/Collections/Collection.php

@@ -488,7 +488,7 @@ public function groupBy($groupBy, $preserveKeys = false)
             }
 
             foreach ($groupKeys as $groupKey) {
-                $groupKey = is_bool($groupKey) ? (int) $groupKey : $groupKey;
+                $groupKey = is_bool($groupKey) ? (int) $groupKey : (string) $groupKey;
 
                 if (! array_key_exists($groupKey, $results)) {
                     $results[$groupKey] = new static;