'upsert' option not working on Model classes

Upsert does not create record if it doesn't exist when called on Model class.

This works:

```
DB::collection('my_collection')->where('name', 'John')->update(['age' => 25], ['upsert' => true]);
```

That doesn't:

```
MyCollection::where('name', 'John')->update(['age' => 25], ['upsert' => true]);
```

`MyCollection` extends `Jenssegers\Mongodb\Model` and `$collection` property is 'my_collection'.
