MINOR: mux-h1: Use htx version to send default low-level errors

When an error was returned by the H1 multiplexer, the raw version was used
for default low-level errors. It is not really an issue, but it is more
consitent to use the HTX version. This way, by default, header names for
such errors are now sent in lower case. And the case of header names can
still be adjusted if necessary, thanks to the previous fix.

It is not really a bug, there is no reason to backport it, except if someone
ask for it.