support pyspark.sql.types.StringType column (#4329)
support pyspark.sql.types.StringType column (#4329)

Co-authored-by: nyamashi <a29d17a5c3dae4be90de95437c93771a53f2ea92@yahoo-corp.jp>
intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL