[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 have also no idea what is this "XJ015"... i taked it from https://github.com/apache/spark/blob/master/core/src/test/scala/org/apache/spark/rdd/JdbcRDDSuite.scala :P