[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

sql/core/src/main/scala/org/apache/spark/sql/api/java/JavaSchemaRDD.scala

@@ -47,6 +47,9 @@ class JavaSchemaRDD(
 
   private[sql] val baseSchemaRDD = new SchemaRDD(sqlContext, logicalPlan)
 
+  /** Returns the underlying Scala SchemaRDD. */
+  val schemaRDD: SchemaRDD = baseSchemaRDD
+
   override val classTag = scala.reflect.classTag[Row]
 
   override def wrapRDD(rdd: RDD[Row]): JavaRDD[Row] = JavaRDD.fromRDD(rdd)