Use correct type when storing preferences.

We should not use the value, but the key. Value stores descriptions,
while key is actually the thing stored in settings.

libraries/config/FormDisplay.class.php

@@ -468,7 +468,7 @@ class FormDisplay
             // (allows to skip 0 == 'string' equalling to true) or identity (for string-string)
             if (($vk == $value && !(is_numeric($value_cmp) xor is_numeric($vk)))
                     || $vk === $value) {
-                settype($value, gettype($v));
+                settype($value, gettype($vk));
                 return true;
             }
         }