[SPARK-911] allow efficient queries for a range if RDD is partitioned wi...

...th RangePartitioner
@JoshRosen maybe it's my version of scala or something, but the code won't compile without it. It complains that it found `Iterator[(K,V)]` but required `Iterator[P]`, which is odd because those should be identical types. If you think that the code will compile without it on Jenkins I can push without the cast