[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


changed to while, learned to write from your ```fillObject``` in hive ```TableReader``` :D https://github.com/apache/spark/blob/2b8d89e30ebfe2272229a1eddd7542d7437c9924/sql/hive/src/main/scala/org/apache/spark/sql/hive/TableReader.scala#L279-L288

thanks.