Fix for private chef group add actor for 'user'

Private chef bombs on the ID for users and the fact it wants to check on clients/groups.  Not sure if this is the right way to do this, but it works for our private chef installation.  Current knife-acl bomps like so for 'knife group add actor admins username':

INFO: HTTP Request Returned 500 Internal Server Error: Client pivotal does not exist
ERROR: Server returned error for https://chefserver/organizations/infrastructure-56m/groups/admins, retrying 3/5 in 13s

If I just remove the clients section, I get:
INFO: HTTP Request Returned 500 Internal Server Error: Users 3029dc16617811e298d590b11c089a5b do not exist
ERROR: Server returned error for https://chefserver/organizations/infrastructure-56m/groups/admins, retrying 1/5 in 4s

It seems private chef wants the username, not the ID?  This works well against a test installation, but wanted to get an official review before using it.