Action.Submit "data" property as string

Re #652

The docs for Action.Submit today say that data can be a string or object. WebChat currently uses a string to trigger an "imBack" to the bot, and they use data as an object to post back more complicated data.

http://adaptivecards.io/explorer/Action.Submit.html

The following currently causes nothing to happen when I press submit. This seems like a recent bug?

I believe the expected response would be:

{
 "data": "hello",
 "firstName": "",
 "lastName": ""
}

Payload:

{
    "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
    "type": "AdaptiveCard",
    "version": "1.0",
    "body": [
        {
            "type": "Input.Text",
            "id":"lastName",
            "placeholder": "What is your last name?"
        }
    ],
    "actions": [
        {
            "type": "Action.Submit",
            "title": "Action.Submit",
            "data": "hello"
        }
    ]
}

This payload works:

            "data": {
                "x" : 13
            }      

Start by reproducing the supplied Adaptive Card payload and compare it with the working object-valued Action.Submit.data example. Read the Action.Submit behavior described in the linked explorer documentation; done means a string-valued data submission produces the expected response containing data, firstName, and lastName.