Custom query helpers on embedded schemas

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

**What is the current behavior?**
Query helpers defined on schemas which are embedded into other schemas are not transferred to the parent schema.

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

https://gist.github.com/LK/3375e8199fccc22b70817842b1a570ef

**What is the expected behavior?**
The query helper from the embedded document is inherited into the parent schema.

**Please mention your node.js, mongoose and MongoDB version.**
Node: 9.5.0
Mongoose: 5.1.4
MongoDB: 3.4.2

Start with the linked reproduction gist and trace how query helpers are registered on an embedded schema and exposed through the parent schema. Confirm the expected inheritance behavior against the reported Node, Mongoose, and MongoDB versions; done means the embedded helper is available when querying through the parent schema and the reproduction succeeds.

Written by the indexing model from the issue text.