Validators should work for nested objects

(From https://github.com/Automattic/mongoose/issues/3587#issuecomment-184918722)

Validators on nested objects currently don't work. Using subdocuments works around this for now, but it would be nice if directly using nested objects would work.

For example, using this test case:

``` js
"use strict";

const mongoose = require("mongoose");
mongoose.connect("localhost/test");

const schema = new mongoose.Schema({
a: {
$type: {
a: {
$type: String,
required: true
}
},
required: true
}
}, {
strict: "throw",
typeKey: "$type"
});

const Test = mongoose.model("Test", schema);

// Should throw for missing `a.a` and extra `a.b`
Test.create({a: {b: "test"}}).then(function (record) {
console.log(record.validateSync());
console.log(record);
}).catch(e => console.log(e));
```

throws:

```
TypeError: Undefined type `undefined` at `a.$type.a.required`
Did you try nesting Schemas? You can only nest using refs or arrays.
at Function.Schema.interpretAsType (/tmp/node_modules/mongoose/lib/schema.js:598:11)
```

Reproduce the nested-object example, then inspect lib/schema.js around Schema.interpretAsType and line 598, where the reported error occurs. Confirm that directly nested validators accept the schema definition, enforce missing a.a, and still reject extra a.b under strict mode.

Written by the indexing model from the issue text.