Fix #3894: Allow loading pickles without self.booster attributes (redux) (#3944)
Fix #3894: Allow loading pickles without self.booster attributes (redux) (#3944)


dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost