cfg_cb auth failure when bucketname != username

When attempting to connect to an RBAC-secured bucket, when username != bucketname, cfg_cb is failing to connect, with an error like:

2017/05/23 13:52:34 cfg_cb: OnError, err: HTTP error 401 Unauthorized getting "http://localhost:8091/pools": 

2017/05/23 13:52:34 cfg_cb: OnError, err: could not connect to any serverURL: []string{"http://accel:password@localhost:8091"}

The problem may be in CfgCB's implementation of GetCredentials - it looks like it's always returning username as bucketname, even if the user is defined in the url.