[SPARK-2710] [SQL] Build SchemaRDD from a JdbcRDD with MetaData

SPARK-2710 Build SchemaRDD from a JdbcRDD with MetaData

and a small bug fix on JdbcRDD, line 109
it seems conn will never be closed


i find a easier way, we can check rs.wasNull after the dataType match {}, then just once
if (rs.wasNull) row.update(i, null)