Make subdomain validation more restrictive

- Each subdomain part must have a length no greater than 63.
- Each subdomain part must begin and end with an alpha-numeric (i.e. letters [A-Za-z] or digits [0-9]).
- Each subdomain part may contain hyphens (dashes), but may not begin or end with a hyphen.

src/Zendesk/API/Utilities/OAuth.php

@@ -83,7 +83,7 @@ public static function getAuthUrl($subdomain, array $options, $domain = 'zendesk
      */
     private static function validateSubdomain($subdomain)
     {
-        if (! preg_match('/^[0-9A-Za-z-]+$/', $subdomain)) {
+        if (! preg_match('/^[A-Za-z0-9](?:[A-Za-z0-9\-]{0,61}[A-Za-z0-9])?$/', $subdomain)) {
             throw new InvalidArgumentException('Invalid Zendesk subdomain.');
         }
     }