Bug fix: pubsub message's data and label fields missing in publishRaw().

The official API documentation for `publish` endpoint requires `data` and `label` nested under `message` key of the request body. (see https://cloud.google.com/pubsub/v1beta1/topics/publish)

```js
{
  "topic": string,
  "message": {
    "data": bytes,
    "label": [
      {
        "key": string,
        "strValue": string,
        "numValue": long
      }
    ]
  }
}
```

The current code builds an incorrect request body:

```js
{
  "topic": string,
  "data": bytes,
  "label": [
    {
      "key": string,
      "strValue": string,
      "numValue": long
    }
  ]
}
```

This PR fixes this bug.