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