_id must go first + test for updating accross indexes

Hi,

MongoDB store _id first when inserting values : 

```javascript
connecting to: test
> db.fongo.remove();
> db.fongo.insert({date:1, _id:1})
> db.fongo.find();
{ "_id" : 1, "date" : 1 }
```

+ one test to check if update works againts indexes.