BUG/MINOR: h1-htx: Use default reason if not set when formatting the response

BUG/MINOR: h1-htx: Use default reason if not set when formatting the response

When the response status line is formatted before sending it to the client,
if there is no reason set, HAProxy should add one that matches the status
code, as stated in the configuration manual. However it is not performed.

It is possible to hit this bug when the response comes from a H2 server,
because there is no reason field in HTTP/2 and above.

This patch should fix the issue #2798. It should be backported to all stable
versions.