Domain zone serial not updated when adding rr via remote API

short description

When using the remote API (/remote/json.php) to add a new resource record to the DNS server (e. g. a new TXT record), the record is added perfectly fine, but the zone serial number is not incremented. This results in the update not being transfered to the zone's slave DNS servers which makes DNS server changes using the remote API useless.

correct behaviour

The domain zone serial number should be automatically incremented after adding/changing/removing an resource record via the remote API.

environment

Server OS: Debian (but should occor on any)
Server OS version: jessie (but should occur on any) ISPConfig version: 3.1.11