SPARK-1663. (Addendum) Fix signature of one version of JavaPairRDDStream.reduceByKeyAndWindow()

It appears that one of these methods doesn't use `org.apache.spark.api.java.function.Function2` like all the others, but uses Scala's `Function2`.

Author: Sean Owen <sowen@cloudera.com>

Closes #633 from srowen/SPARK-1663.2 and squashes the following commits:

1e0232d [Sean Owen] Fix signature of one version of reduceByKeyAndWindow to use Java API Function2, as apparently intended

streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaPairDStream.scala

@@ -262,7 +262,7 @@ class JavaPairDStream[K, V](val dstream: DStream[(K, V)])(
    * @param windowDuration width of the window; must be a multiple of this DStream's
    *                       batching interval
    */
-  def reduceByKeyAndWindow(reduceFunc: Function2[V, V, V], windowDuration: Duration)
+  def reduceByKeyAndWindow(reduceFunc: JFunction2[V, V, V], windowDuration: Duration)
   :JavaPairDStream[K, V] = {
     dstream.reduceByKeyAndWindow(reduceFunc, windowDuration)
   }