I've opened a PR [1] that adds a decent amount of client-based test coverage for the reactive extensions. As I mentioned in the commit message, these tests indicate the existence of at least two bugs that need to be addressed. In the case of `testSequentialHeadRequests`, I have additional changes (which I will also publish shortly) that fixes this issue for non-minimal clients, but I have yet to fix the issue on minimal clients. For the other, non-deterministic failure, I don't have a fix yet, and I may need help finding the root cause.
[1] https://github.com/apache/httpcomponents-client/pull/181
