Bug fix: pubsub does not publish message data.

The official API documentation for `publish` endpoint requires `data` and `label` nested under `message` key. (see https://cloud.google.com/pubsub/v1beta1/topics/publish)

```json
{
  "topic": string,
  "message": {
    "data": bytes,
    "label": [
      {
        "key": string,
        "strValue": string,
        "numValue": long
      }
    ]
  }
}
```