MINOR: http-htx: Handle an optional reason when replacing the response status

MINOR: http-htx: Handle an optional reason when replacing the response status

When calling the http_replace_res_status() function, an optional reason may now
be set. It is ignored if it points to NULL and the original reason is
preserved. Only the response status is replaced. Otherwise both the status and
the reason are replaced.

It simplifies the API and most of time, avoids an extra call to
http_replace_res_reason().