NODE-4221 Description

How are you using Mongo? What version of the server and driver are you using?

Have a client-side application where I want to serialize raw data using the extended-json in order to send data to the backend. That data contains objects with explicit undefined values. I also may have a case where the data contains explicit null values

{zig: "zag", foo: undefined, bar: null}

What is the feature/improvement you would like?

I would like to be able to serialize the above with EJSON.stringify to 

{zig: "zag", bar: null}

Ideally the EJSON options includes an `ignoreUndefined` property like the regular serialize function.

What use case would this feature/improvement enable?

More explicit intention in encoding/serializing with the EJSON API

Current workaround is to have some singleton represent values I would actually want as null, and have a replacer handle converting them to null and excluding undefines.