BUG: counters: third counter was not stored if others unset

Commit e25c917a introduced a third tracking counter bug forgot
to check it when storing values at the end of the session. The
impact is that  if neither the first nor the second one are
changed, none of them are saved.
DOC: minor typo fix in documentation

"http-reqsponse" => "http-response"

Signed-off-by: Godbach <nylzhaowei@gmail.com>