Throws an error when votes is pruned by projection

I want to add a projection to my query to return minimal data, but when votes array is missing the plugin crashes

This query works:
`Model.find(query, {field: 1, vote:1})`

This does not:
`Model.find(query, {field: 1})`

```
TypeError: Cannot read property 'length' of undefined
    at model.upvotes (node_modules\mongoose-voting\index.js:95:30)
```
