AdaptiveCards: [Beta10] Context.PostAsync not rendering AdditionalProperties of AdaptiveCard

After updating the nuget package “AdaptiveCards” to version=“1.0.0-beta09” the code below has no efect on the json returned by my bot:

var card = new AdaptiveCard();
card.AdditionalProperties.Add(new KeyValuePair<string, object>("customType", "MyCard1"));

It seem that after calling the AdditionalProperties.Add() method has no efect.