Correct inner index

Add errors when creating index like : 

```javascript
> db.fongo.ensureIndex({a:{n:1}});
{
	"err" : "bad index key pattern { a: { n: 1.0 } }",
	"code" : 10098,
	"n" : 0,
	"connectionId" : 256,
	"ok" : 1
}
```