Populate the actors when POSTing groups

When attempting to use knife-ec-restore to groups we found that the clients were not being populated.  After using MITMProxy to see the payload we discovered that the actors were not populated.

```json
{
  "clients": [
    "myorg-validator"
  ],
  "groupname": "validators",
  "groups": [

  ],
  "name": "validators",
  "orgname": "myorg",
  "users": [

  ]
}
```

When we posted with the clients in the actors all is well

```shell
root@test-standalone:/tmp# knife exec -E 'pp api.post("groups", { groupname: "qux", actors: { clients: ["myorg-validator"]}})'
{"uri"=>"https://localhost/organizations/myorg/groups/qux"}
root@test-standalone:/tmp# knife exec -E 'pp api.get("groups/qux")'
{"actors"=>["myorg-validator"],
 "users"=>[],
 "clients"=>["myorg-validator"],
 "groups"=>[],
 "orgname"=>"myorg",
 "name"=>"qux",
 "groupname"=>"qux"}
```