`withCache` can send stale data

[benweitzman]

1. Create an underlying observable
2. Wrap using withCache
3. Attach an observer to the cached observable
4. Send a value (A) through the underlying observable
5. Unobserve from the cached observable
6. Send another value (B) through the underlying
7. Attach a new observer to the cached observable
8. New observer receives (A) immediately, despite A being a stale value.

This issue is made worse for observers like toPromise which will unobserved after receiving their first value, meaning they only ever receive a stale value.