[SPARK-14671][ML] Pipeline setStages should handle subclasses of PipelineStage

## What changes were proposed in this pull request?

Pipeline.setStages failed for some code examples which worked in 1.5 but fail in 1.6.  This tends to occur when using a mix of transformers from ml.feature. It is because Java Arrays are non-covariant and the addition of MLWritable to some transformers means the stages0/1 arrays above are not of type Array[PipelineStage].  This PR modifies the following to accept subclasses of PipelineStage:
* Pipeline.setStages()
* Params.w()

## How was this patch tested?

Unit test which fails to compile before this fix.

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #12430 from jkbradley/pipeline-setstages.