Optional sort param

Hi @diegohaz, I'm thinking about mandatory sort. We are working with MongoDb, and some querys we don't need an index for sorting, but the "sort" param is always "one" because we have a default value:

```
sort: {
type: [String],
default: '-createdAt',
bindTo: 'cursor',
parse: (value) => {
let fields = _.isArray(value) ? value : [value]
let sort = {}
fields.forEach((field) => {
if (field.charAt(0) === '-') {
sort[field.slice(1)] = -1
} else if (field.charAt(0) === '+') {
sort[field.slice(1)] = 1
} else {
sort[field] = 1
}
})
return {sort: sort}
}
}
```

Can you please give some "light" to try this sort param in the Schema as "optional". Thanks!