[SPARK-16426][MLLIB] Fix bug that caused NaNs in IsotonicRegression

## What changes were proposed in this pull request?

Fixed a bug that caused `NaN`s in `IsotonicRegression`. The problem occurs when training rows with the same feature value but different labels end up on different partitions. This patch changes a `sortBy` call to a `partitionBy(RangePartitioner)` followed by a `mapPartitions(sortBy)` in order to ensure that all rows with the same feature value end up on the same partition.

## How was this patch tested?

Added a unit test.

Author: z001qdp <Nicholas.Eggert@target.com>

Closes #14140 from neggert/SPARK-16426-isotonic-nan.