DeprecationWarning raised when using a find options string

`collection.find({}, 'name' )` raises a DeprecationWarning. I see #303/#304 fixed documentation by suggesting shifting to projection, rather than fields. This will no doubt work, but the documentation changes do not appear to fully reflect reality when using an options string.

`findOne` [arguments ](https://automattic.github.io/monk/docs/collection/findOne.html#arguments)documentation:
> 2. [`options`] *(Object|String|Array)*: If the `options` is a string, it will be parsed as the projections to select.

`findOne` [example](https://automattic.github.io/monk/docs/collection/findOne.html#example):
```js
users.findOne({name: 'foo'}, 'name').then((doc) => {
// only the name projection will be selected
});
```

Using `monk` 7.3.0, as expected `findOne` with `fields` raises a `DeprecationWarning`:
```
> collection.find({name: 'Test'}, { fields: { 'name': 1 } } );
> (node:125878) DeprecationWarning: collection.find option [fields] is deprecated and will be removed in a later version.
```

`findOne` with `projections` works with no issues:
```
> collection.findOne({name: 'Test'}, { projection: { 'name': 1 } } );
>
```

However, `findOne` appears to continue to parse an options string as `fields`, not `projection` as it raises an unexpected `DeprecationWarning`:
```
> collection.findOne({name: 'Test'}, 'name' );
> (node:126116) DeprecationWarning: collection.find option [fields] is deprecated and will be removed in a later version.
```

The same behaviour is experienced with `find`.

Is the `options` string expansion to `fields` done within `monk` or within mongodb? Either way, the problem should be fixed or the documentation updated to reflect reality.

This also affects the code in [`README.md`](https://github.com/Automattic/monk/blob/master/README.md) which references passing options as a string.

Start by tracing how the find and findOne entry points expand a string options argument, then reproduce the DeprecationWarning described in the issue. Review the linked findOne documentation and README.md for the stated string behavior. Done means either string projections work without the unexpected warning or the documentation accurately describes the observed behavior.

Written by the indexing model from the issue text.