[python-package] Fix the issue #1439 (#1666)
[python-package] Fix the issue #1439 (#1666)

*Fix 1439
        *Fix python_wrapper when eval set name contain '-' will cause early_stop maximize variable con't set to True propely

Change-Id: Ib0595afd4ae7b445a84c00a3a8faeccc506c6d13
dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost