[Form] fix merge

Author: nicolas-grekas

ChoiceList/Loader/AbstractChoiceLoader.php

@@ -59,7 +59,7 @@ public function loadValuesForChoices(array $choices, callable $value = null)
 
         if ($value) {
             // if a value callback exists, use it
-            return array_map(function ($item) use ($value): string { return $value($item); }, $choices);
+            return array_map(function ($item) use ($value) { return (string) $value($item); }, $choices);
         }
 
         return $this->doLoadValuesForChoices($choices);