SPARK-3926 [CORE] Reopened: result of JavaRDD collectAsMap() is not serializable

My original 'fix' didn't fix at all. Now, there's a unit test to check whether it works. Of the two options to really fix it -- copy the `Map` to a `java.util.HashMap`, or copy and modify Scala's implementation in `Wrappers.MapWrapper`, I went with the latter.
It won't have a no-arg constructor in addition to its main one arg constructor otherwise right? This is the point of this change.