Plus path projections underneath nested objects with select: false

Hi! I would like to report which I think may be a bug, or a feature not yet implemented.

I have an object in the schema with a `{ select: false }` opt and I want to select only one field of this object and let everything left hidden.

In other words, I am not able to use the dot notation and the `+` like so:
`.select("+authentification.login")`

to achieve that, the workaround found:
`.select("+authentification -authentification.password");`
but in my case I would have to omit more than one field.

thank you 😃
**standalone strict**
```javascript
const mongoose = require("mongoose");

mongoose.connect("mongodb://localhost:27017/test");

const Test = mongoose.model("Test", {
name: String,
authentification: {
type: {
login: String,
password: String,
},
select: false,
},
});

const data = {
name: "name",
authentification: {
login: "login",
password: "password",
},
};

async function main() {
await Test.create(data);

const userWrong = await Test.findOne({}).select("+authentification.password");
console.log(userWrong);

// workaround to make this work
const userCorrect = await Test.findOne({}).select(
"+authentification -authentification.password",
);
console.log(userCorrect);
}

main();
```
"mongoose": "5.2.18",

No source file or test is named. Start by running the standalone JavaScript reproduction against Mongoose 5.2.18 and trace the query projection handling for the nested select:false path; done means selecting +authentification.login returns that field without requiring the whole object, with a regression test covering the behavior.

Written by the indexing model from the issue text.