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Version

What package version of the SDK are you using.
4.14.2

Describe the bug

Give a clear and concise description of what the bug is.
When sending message using adaptive card that contains a People Picker with dataset, Activity can't be serialized (FlatterningSerializer throws error):

Caused by: java.lang.ClassCastException: class com.fasterxml.jackson.databind.node.ArrayNode cannot be cast to class com.fasterxml.jackson.databind.node.ObjectNode (com.fasterxml.jackson.databind.node.ArrayNode and com.fasterxml.jackson.databind.node.ObjectNode are in unnamed module of loader 'app')
at com.microsoft.bot.restclient.serializer.FlatteningSerializer.serialize(FlatteningSerializer.java:187) ~[bot-connector-4.14.2.jar:4.14.2]
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480) ~[jackson-databind-2.13.3.jar:2.13.3]
... 27 common frames omitted

To Reproduce

Steps to reproduce the behavior:
1 Prepare an adaptive card containing People Picker:

    {
      "type": "Input.ChoiceSet",
      "choices": [],
      "choices.data": {
        "type": "Data.Query",
        "dataset": "graph.microsoft.com/users?scope=currentContext"
      },
      "isMultiSelect": true,
      "placeholder": "Choose at least one person for this feedback",
      "label": "Request to",
      "style": "filtered",
      "id": "userId"
    },

2. Send card as message
3. See error in log

Expected behavior

Give a clear and concise description of what you expected to happen.
Expect adaptive card to be sent to chat

Screenshots

If applicable, add screenshots to help explain your problem.

Additional context

Add any other context about the problem here.
The same adaptive card works for Task Module.
The problem is in FlatterningSerializer, where it tries to flat "choices.data", how ever, this can't be done, for the expected flatterned data is:

choices: {
  data: {..}
}

but there's already a choices: [] existing. These two can't match.