only allow closures, fixes #2424

Closes #2424

Merge request studip/studip!1622

lib/classes/JsonApi/JsonApiIntegration/FieldsetFilter.php

@@ -36,7 +36,7 @@ class FieldsetFilter extends \Neomerx\JsonApi\Representation\FieldSetFilter
      */
     private function resolveValue($value)
     {
-        if (!is_string($value) && is_callable($value)) {
+        if ($value instanceof \Closure) {
             return $value();
         }