Oneof toObject loses case

What version of protobuf and what language are you using?
Version: master/v3.6.0/v3.5.0 etc.
Language: Javascript

What operating system Linux, Windows and version?

What runtime / compiler are you using (e.g., python version or gcc version)

What did you do?
Steps to reproduce the behavior:
generated classes and methods via protoc-grpc-web result in oneof losing its case when converted to a plain object by toObject()
See https://github.com/grpc/grpc-web/issues/675 for details

What did you expect to see
The result object to only have one defined property for the oneof case

What did you see instead?
All of the oneof cases are generated in the result plain object with their type default values

Make sure you include information that can help us debug (full error message, exception listing, stack trace, logs).

Anything else we should know about your project / environment

Beginne mit den von protoc-grpc-web generierten Klassen und ihrer toObject()-Konvertierung und verwende das in Issue #675 beschriebene Verhalten als Reproduktionsreferenz. Erledigt ist die Aufgabe, wenn ein einfaches Objekt nur die definierte Eigenschaft für den aktiven oneof-Fall enthält, statt Eigenschaften mit Standardwerten für jeden Fall.