its kind of an implementation detail: `com.ning.http.client.FluentStringsMap`, which is used to serialize the data for a POST request, has an `addAll()` method that takes `Map<String, Collection<String>>`. So the choice is to either transform the `SessionProperties` map in the `com.opentok.util.HttpClient` class, or for the `toMap()` method to return the map in this form. I chose the latter.