Ensure IsExecuting is false when command completes

Because ReactiveCommand decrements the in flight count in a Finally
block, when we await a command's ExecuteAsync method, it still reports
an in-flight count when the await returns. We want to not return until
the in-flight count is decremented. This fixes that.
Merge pull request #714 from reactiveui/haacked/await-executeasync-race-condition-fix

Ensure IsExecuting is false when command completes