BUG/MINOR: http-check: Don't set HTX_SL_F_BODYLESS flag with a log-format body

BUG/MINOR: http-check: Don't set HTX_SL_F_BODYLESS flag with a log-format body

When the HTTP request of a health-check is forged, we must not pretend there
is no payload, by setting HTX_SL_F_BODYLESS, if a log-format body was
configured.

Indeed, a test on the body length was used but it is only valid for a plain
string. For A log-format string, a list is used. Note it an bug with no
consequence for now.

This patch must be backported as far as 2.2.