Subscription field alias is ignored

In the recent #844 the alias field is ignored:

````
subscription NewMessages { 
    newsFeed: newMessage(roomId: 123) { 
         sender 
         text 
    }
}
````
Expected result would be:
````
{ 
    "newsFeed": { 
         "sender": ".....", 
         "text": "...." 
    } 
}
````
