BUG/MINOR: mux-h2: use timeout http-request as a fallback for http-keep-alive

BUG/MINOR: mux-h2: use timeout http-request as a fallback for http-keep-alive

The doc states that timeout http-keep-alive is not set, timeout http-request
is used instead. As implemented in commit  15a4733d5 ("BUG/MEDIUM: mux-h2:
make use of http-request and keep-alive timeouts"), we use http-keep-alive
unconditionally between requests, with a fallback on client/server. Let's
make sure http-request is always used as a fallback for http-keep-alive
first.

This needs to be backported wherever the commit above is backported.

Thanks to Christian Ruppert for spotting this.