title: mongo-go-driver aggregate query always return "Current": null
body: <p>I want to sum a field by using </p>
  
  <pre><code>pipeline := []bson.M{
      bson.M{"$group": bson.M{
          "_id": "", 
          "count": bson.M{ "$sum": 1}}}
  }
  ctx, _ := context.WithTimeout(context.Background(), 5*time.Second)
  result, err := collection.Aggregate(ctx, pipeline)
  </code></pre>
  
  <p>but it always return</p>
  
  <pre><code>"Current": null
  </code></pre>
  
  <p>Any solution?</p>