Update Serbian postal code validation

According to Wikipedia, also known as the most reliable source of
information on the whole Internet, Serbian postal codes consist of five
digits [1].

Even though it mentions that a six-digit postal code format has been in
place since 1 January 2005 [2], I have not found any Serbian postal code
which consists of six digits.

Maybe someone would prove us wrong, but we will keep the validation as
six-digits from now one.

[1]: https://en.wikipedia.org/wiki/Postal_codes_in_Serbia
[2]: https://web.archive.org/web/20090917083620/http://www.posta.rs/postanskamreza/postanskamreza.asp

Co-authored-by: Henrique Moody <henriquemoody@gmail.com>

library/Rules/PostalCode.php

@@ -156,7 +156,7 @@ final class PostalCode extends AbstractEnvelope
         'PY' => '/^(\d{4})$/',
         'RE' => '/^((97|98)(4|7|8)\d{2})$/',
         'RO' => '/^(\d{6})$/',
-        'RS' => '/^(\d{6})$/',
+        'RS' => '/^(\d{5})$/',
         'RU' => '/^(\d{6})$/',
         'SA' => '/^(\d{5})$/',
         'SD' => '/^(\d{5})$/',

tests/unit/Rules/PostalCodeTest.php

@@ -88,6 +88,7 @@ final class PostalCodeTest extends RuleTestCase
             [new PostalCode('KY'), 'KY2-2001'],
             [new PostalCode('KY'), 'KY3-2500'],
             [new PostalCode('AM'), '0010'],
+            [new PostalCode('RS'), '24430'],
         ];
     }
 
@@ -111,6 +112,7 @@ final class PostalCodeTest extends RuleTestCase
             [new PostalCode('EC'), 'A1234B'],
             [new PostalCode('KY'), 'KY4-2500'],
             [new PostalCode('AM'), '375010'],
+            [new PostalCode('RS'), '244300'],
         ];
     }
 }