[SPARK-4262][SQL] add .schemaRDD to JavaSchemaRDD

 marmbrus

Author: Xiangrui Meng <meng@databricks.com>

Closes #3125 from mengxr/SPARK-4262 and squashes the following commits:

307695e [Xiangrui Meng] add .schemaRDD to JavaSchemaRDD

(cherry picked from commit 3d2b5bc5bb979d8b0b71e06bc0f4548376fdbb98)
Signed-off-by: Xiangrui Meng <meng@databricks.com>