[web request] fix cookie 'expires' formating (closes #1953945)
This was because cookie.expires wasn't processed in cw.etwist.http,
though this code should had never existed, instead proper twisted
method should be called.
Also, move on the way to headers handling simplification and rewrite
cw request.set_cookie / remove_cookie to directly use the Cookie class
in cw.web.http_headers rather than going back and forth
simple cookie <-> raw string <-> http_headers.Cookie
IMO more on this should be done.