Reject promise on .create() error

Promise was not rejected on .create() error.
Example (trying to duplicate "unique" field):
```
var m = Model.create({ _id: 'already_used' });
m.then(funcion(doc) {
  Model.create({ _id: 'already_used' })
    .then(function () {}, function (err) { //never called });
});
```

Thanks!! Mongoose is awesome ;)