cookie: case-insensitive path comparison in replace_existing() allows cookie eviction across distinct paths

`replace_existing()` in `lib/cookie.c` compares cookie **paths** case-insensitively at two sites. On case-sensitive servers, `/Admin` and `/admin` are distinct resources and are supposed to produce distinct jar entries. Because libcurl conflates them, a `Set-Cookie` at one path silently evicts the …

if(curl_strnequal(clist->path, co->path, cllen)) {
  ...
}