default writeConcert to ACKNOWLEDGED

Hi,

the new "Fongo" has a writeConcert to "null".

Add two test from real life, if you know why 

```javascript
db.fongo.insert({_id:1})
db.find({id:1}).skip(1).toArray()  => return {_id:1}
```

it will help me..

I suspect an "index scan only", I'm not sure. any idea ?