The response of `db.collection.insert` have malformed `insertedIds` properties.

Here is a sample of what I got:

```
{ result:

,
ops:
[

],
insertedCount: 1,
insertedIds: [ [ '0' ], 567c09f017e825c31ee849da ] }
```

And I traced down the source code, I found a possible reason for this result.

1. The bulkWrite function already maps the raw response to an object with index being the key and object id being the value. https://github.com/mongodb/node-mongodb-native/blob/2.0/lib/collection.js#L605
2. In insertMany which invoked by insert, mapInInsertManyResult is called. https://github.com/mongodb/node-mongodb-native/blob/2.0/lib/collection.js#L486
3. As for mapInInsertManyResult, the same mapping happens again. https://github.com/mongodb/node-mongodb-native/blob/2.0/lib/collection.js#L411