Allow default affiliation in Register

[vdods]

The Fabric-CA docs https://hyperledger-fabric-ca.readthedocs.io/en/latest/users-guide.html#registering-a-new-identity say If no affiliation is specified in the registration request, the identity being registered will be given the affiliation of the registrar. but there is a check in caclient.go Register for if the affiliation is set in the request. I've made the following change in my fork to allow this.

--------------------------------- caclient.go ---------------------------------
index f572651..8ea697c 100644
@@ -417,9 +417,9 @@ func (f *FabricCAClient) Register(identity *Identity, req *CARegistrationRequest
if req.EnrolmentId == "" {
return "", ErrEnrolmentMissing
}

• if req.Affiliation == "" {

•   return "", ErrAffiliationMissing
  

• }
  +// if req.Affiliation == "" {
  +// return "", ErrAffiliationMissing
  +// }
  if req.Type == "" {
  return "", ErrTypeMissing
  }

I'm happy to make a PR for this if it makes things easier.