Netplan does not add default route for ipv6 interface with static IP

[AD]

As title, netplan does not add a default route for my `gateway6` directive in its config file. The specified IP address for the interface is correctly applied

Using configuration as follows on ubuntu server 18.04.2:
```
network:
    version: 2
    ethernets:
        ens3:
            addresses:
                - "2001:41d0:801:xxxx:0:0:0:17d3/64"
            gateway6: "2001:41d0:0801:xxxx:0000:0000:0000:0001"
            dhcp4: true
            match:
                macaddress: xx:xx:xx:xx:xx:xx
            set-name: ens3
```

after running `netplan aply` I'd expect a default route added for v6 traffic, such as:

```
2001:41d0:801:xxxx::1 dev ens3 metric 1024 pref medium
2001:41d0:801:xxxx::17d3 dev ens3 proto kernel metric 256 pref medium
fe80::/64 dev ens3 proto kernel metric 256 pref medium
default via 2001:41d0:801:xxxx::1 dev ens3 metric 1024 pref medium
```

Instead, `ip -6 route show` returns:
```
2001:41d0:801:xxxx::17d3 dev ens3 proto kernel metric 256 pref medium
fe80::/64 dev ens3 proto kernel metric 256 pref medium
```