observeOnce did not unsubscribe correctly

[compulim]

Background

https://github.com/Microsoft/BotFramework-WebChat/blob/master/packages/core/src/sagas/effects/observeOnce.js#L11

In this line of code, we didn't assign the result of observable.subscribe() to subscription. Thus, subscription is always empty and thus, we never call unsubscribe().

Things to-do

1. Modify https://github.com/Microsoft/BotFramework-WebChat/blob/master/packages/core/src/sagas/effects/observeOnce.js#L11
   • Assign the result of observable.subscribe() to subscription variable
2. Verify unsubscribe() is being called correctly
3. Verify all callers of observeOnce will continues to work