BUG/MEDIUM: http: fix incorrect reporting of server errors

BUG/MEDIUM: http: fix incorrect reporting of server errors

Commit dbe34eb ("MEDIUM: filters/http: Move body parsing of HTTP
messages in dedicated functions") introduced a bug in function
http_response_forward_body() by getting rid of the while(1) loop.
The code immediately following the loop was only reachable on missing
data but now it's also reachable under normal conditions, which used
to be dealt with by the skip_resync_state label returning zero. The
side effect is that in http_server_close situations, the channel's
SHUTR flag is seen and considered as a server error which is reported
if any other error happens (eg: client timeout).

This bug is specific to 1.7, no backport is needed.