$TTL in zone definition is not updated

[Impact]

The default TTL for a zone is not updated when the ttl is changed

[Steps to reproduce]

$ maas maas domain update 0 ttl=600
Success.
Machine-readable output follows:
{
    "name": "maas",
    "id": 0,
    "authoritative": true,
    "resource_record_count": 3,
    "resource_uri": "/MAAS/api/2.0/domains/0/",
    "ttl": 600
}

Expected result:
⟫ cat /etc/bind/maas/zone.maas
; Zone file modified: 2017-09-22 17:42:34.180183.
$TTL 600
@ IN SOA maas. nobody.example.com. (
              0000000803 ; serial
              600 ; Refresh
              1800 ; Retry
              604800 ; Expire
              30 ; NXTTL
              )

@ 600 IN NS maas.
$GENERATE 191-254 192-168-11-$ IN A 192.168.11.$
$GENERATE 190-253 192-168-12-$ IN A 192.168.12.$
virbr0.maas-rackd 600 IN A 192.168.122.1
...

Actual result:
⟫ cat /etc/bind/maas/zone.maas
; Zone file modified: 2017-09-22 17:42:34.180183.
$TTL 30
@ IN SOA maas. nobody.example.com. (
              0000000803 ; serial
              600 ; Refresh
              1800 ; Retry
              604800 ; Expire
              30 ; NXTTL
              )

@ 600 IN NS maas.
$GENERATE 191-254 192-168-11-$ IN A 192.168.11.$
$GENERATE 190-253 192-168-12-$ IN A 192.168.12.$
virbr0.maas-rackd 600 IN A 192.168.122.1
...

[Other info]

Reproduced with 2.2.0 and 2.3.0-alpha3