Follow-up Issues with empty PollForDecisionTask and PollForActivityTask responses

[mfine]

Following up #257. Finally pulled in the fix for #260 and ran into new serialization issues on more missing fields (sorry for the delay!). Looking at the models, it seems like there's more optional attributes to be added here - the responses outlined in #257 for empty polls are still accurate, meaning more overrides.

Empty PollForDecisionTask

Required:

    "DecisionTask":{
      "type":"structure",
      "required":[
        "taskToken",
        "startedEventId",
        "workflowExecution",
        "workflowType",
        "events"
      ],

Actual:

[Raw Response Body] {
{"previousStartedEventId":0,"startedEventId":0}
}

Overrides:

        "DecisionTask": {
            "optionalFields": [
                "taskToken",
                "workflowExecution",
                "workflowType",
                "events"
            ]

Empty PollForActivityTask

Required:

    "ActivityTask":{
      "type":"structure",
      "required":[
        "taskToken",
        "activityId",
        "startedEventId",
        "workflowExecution",
        "activityType"
      ],

Actual:

[Raw Response Body] {
{"startedEventId":0}
}

Overrides:

            "optionalFields": [
                "activityId",
                "taskToken",
                "workflowExecution",
                "activityType"
            ]

[brendanhay]

Thanks - closing as I assume #286 + f15725e has this covered.