Schema.path('path to object') returns undefined

**Do you want to request a *feature* or report a *bug*?**

bug

**What is the current behavior?**

`MySchema.path('path to object')` returns `undefined`.

**If the current behavior is a bug, please provide the steps to reproduce.**

Having:
```ts
const UserSchema = new Schema({
test1: {
a: String
},
test2: {
type: {
a: String
}
},
test3: [{
a: String
}]
});
```
Will make
`MySchema.path('test1')` == `undefined`.

While the other ones work:
`MySchema.path('test2')` != `undefined`.
`MySchema.path('test3')` != `undefined`.

**What is the expected behavior?**

I would expect `test1` to work the same way as `test2`.

**Please mention your node.js, mongoose and MongoDB version.**
Node v11.6.0
Mongoose: 5.4.3
MongoDB: v4.0.4

Start by reproducing the behavior with the supplied UserSchema definition and the three Schema.path() calls, using the reported Node, Mongoose, and MongoDB versions if available. Trace how Schema.path() handles nested objects versus typed objects and arrays; done means test1 resolves consistently with test2 and test3, with coverage for the reported shape.

Written by the indexing model from the issue text.