Fix bug preventing defaultMinSplits from being set >2 (SPARK-822)

This fixes [SPARK-822](https://spark-project.atlassian.net/browse/SPARK-822), an issue where defaultMinSplits would be capped at 2 even if `spark.default.parallelism` was set higher.  This could cause jobs to not use enough parallelism if they used the default settings.

The problem was the use of `min` instead of `max`.