4.x: FirstAsync don't create exception after finding the item

[akarnokd]

When FirstAsync finds an item, it disposes the upstream, however, the upstream may still emit an OnCompleted if it is unable to detect this dispose call (or ignores it). In this case, the FirstAsync's OnCompleted gets invoked which creates an exception that gets thrown away as the base observer is already the no-op observer, wasting resources creating the stacktrace in the process. Both plain and predicated versions are affected.

I don't know how to verify this via unit test, but this extra call can be seen when placing a breakpoint in FirstAsync's OnCompleted method and running this flow:

Observable.Return(1).FirstAsync().Subscribe();