No Notification for Data Message

### Problem description
When I send a 'data' message, no notification is shown if the app isn't open. I took a look at #9, but I'm fairly certain I'm composing the message correctly.

### Expected behavior
The [documentation](https://github.com/eclipsesource/tabris-plugin-firebase/blob/master/doc/cloud-messaging.md) states that a 'data' message is "a notification that also delivers its data to the app", so, I would expect a notification to appear.

### Environment
- Tabris.js version: 3.0.0-beta1
- Device: iPhone 7
- OS: iOS 11.1.1

### Code snippet
##### In-app Javascript:
```javascript
/* check to see if app was launched by notification tap */
setTimeout(() => { // workaround for #41
console.log('launchData after timeout: ', firebase.Messaging.launchData);
});

/* handle any messages that arrive */
firebase.Messaging.on('message', ({data}) => {
console.warn('Received message data: ', data)
});
```
##### Command Line Notification Composition:
```bash
curl -X POST -H "Authorization: key=$key" -H "Content-Type: application/json" -d "{
\"to\": \"$token\",
\"data\": {
\"title\": \"Title\",
\"body\": \"Body\",
\"payload\": \"custom data\"
}
}
" https://fcm.googleapis.com/fcm/send
# Response in terminal: {..."success":1,"failure":0,...}
```
If the app is open, then the following prints:
`Received message data: {payload: 'custom data', title: 'Title', body: 'Body',...}`

But if the app is closed, then nothing prints.