make string conversion in ForumHelper::secure() less specific

see: https://github.com/cakephp/cakephp/pull/6860/files#r32937821

src/View/Helper/FormHelper.php

@@ -557,10 +557,10 @@ class FormHelper extends Helper
         $unlockedFields = $this->_unlockedFields;
 
         foreach ($fields as $key => $value) {
+            if (is_numeric($value)) {
+                $value = (string)$value;
+            }
             if (!is_int($key)) {
-                if (is_numeric($value)) {
-                    $value = (string)$value;
-                }
                 $locked[$key] = $value;
                 unset($fields[$key]);
             }