HTTP healthchecking will be implicitely activated for proxy upstream
hosts, even if not configured. The README states that not using the
health_check directive will disable HTTP healthchecks though.

It seems to me that the availability of the HealthCheck.Path attribute
is used as indicator whether HTTP healthchecks should be used or not.
The normalizeCheckURL() function didn't check that attribute though,
always returning a CheckURL. This would increase the healthcheck failure
on every third failure in plugin/proxy, without any possibility for the
upstream host to be marked as healthy again. This would eventually
remove all upstream hosts from the serving pool.