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.

Author: Sean Owen <sowen@cloudera.com>

Closes #3587 from srowen/SPARK-3926 and squashes the following commits:

8586bb9 [Sean Owen] Remove unneeded no-arg constructor, and add additional note about copied code in LICENSE
7bb0e66 [Sean Owen] Make SerializableMapWrapper actually serialize, and add unit test