BUG/MINOR: http-fetch: do not count Set-Cookie attributes as cookies

BUG/MINOR: http-fetch: do not count Set-Cookie attributes as cookies

A Set-Cookie header contains a single cookie, and other elements on
the same line are not cookies but attributes (which can easily be
mistaken for them by a parser). That's what happens for res.cook_cnt()
which uses the same parser as req.cook_cnt() and counts apparent cookie
values without figuring it's parsing attributes. Let's just do like for
res.cookie_names() and check for a request or response before looping,
so that we can now count a single cookie per set-cookie response.

This can be backported to all stable versions.