The API field `EnableIPv6` was marked as optional in our Swagger docs,
and its default value in the Go client came from that field being a
bool, thus defaulting to its zero value. That's not the case anymore.
This field is now a `*bool` as to let daemon's config define the default
value. IPv6 can still be enabled / disabled by explicitly specifying the
`--ipv6` flag when doing `docker network create`.
Signed-off-by: Albin Kerouanton <albinker@gmail.com>
db2672e685