Typehint for php 8.0 compatibility

Author: peldax

src/Form/ResultObjectSet.php

@@ -35,7 +35,7 @@ public static function fromArray(array $data) : \Nepttune\Form\ResultObjectSet
         return new self($objectSet);
     }
 
-    public function jsonSerialize()
+    public function jsonSerialize() : array
     {
         $toReturn = [];