Added an RDS that repeatedly returns the same input

streaming/src/main/scala/spark/streaming/ConstantInputRDS.scala

@@ -0,0 +1,18 @@
+package spark.streaming
+
+import spark.RDD
+
+/**
+ * An input stream that always returns the same RDD on each timestep. Useful for testing.
+ */
+class ConstantInputRDS[T: ClassManifest](ssc: SparkStreamContext, rdd: RDD[T])
+  extends InputRDS[T](ssc) {
+
+  override def start() {}
+
+  override def stop() {}
+
+  override def compute(validTime: Time): Option[RDD[T]] = {
+    Some(rdd)
+  }
+}