Come up with a better way of setting cookies.

[mattiasgrenfeldt]

Currently, to set cookies on a response you have to use the safehttp.Header.SetCookie method which takes a http.Cookie. All other ways to modify header values block the modification of the Set-Cookie header.

I suggest that we remove the safehttp.Header.SetCookie method, implement a safehttp.Cookie type with safe defaults and add a SetCookie method to safehttp.ResponseWriter.