Fix for MX with priority 0

Author:    Paul Jones <info@beyondthebox.co.nz>
Date:      Sun Jun 21 07:42:38 2020 +0000

src/Endpoints/DNS.php

@@ -56,7 +56,7 @@ class DNS implements API
             $options['ttl'] = $ttl;
         }
 
-        if (!empty($priority)) {
+        if (is_numeric($priority)) {
             $options['priority'] = (int)$priority;
         }