HttpClient5 consistently fails HEAD requests that have the "Connection: close" header on the response because the runtime always attempts to close the IOSession handling the connection twice. The second close triggers a ConnectionClosedException, causing the request to fail.
For some reason, this only affects HEAD requests. All other HTTP methods work fine.
I don't have a repro that I can share yet, but I'm working on a fix. I'm letting you know because I don't know if I'll need to make any changes in core (I suspect I will). Can you wait to release beta9 until I have a root cause?