Problem unearthed during testing of OPENIG-4531.
The strange thing is that this test works when applied to a ReverseProxyHandler but does not when applied to a ClientHandler. Noted that there are additional filters inserted when using the ReverseProxyHandler. Note also that the ClientHandler supports this request as plain HTTP/1.1 and HTTP/2 with HTTPS, but does not seem to manage this as an upgrade.
Unclear if this is a bug - or a docs issue. That is, it may be a docs issue if the ClientHandler does not support HTTP2 upgrade when used with IG Standalone.