Do not use last element of uri path in the default cookie path value

[zellerin]

In get-cookies in cookies.lisp, the :path for cookie is set to

(or (parameter-value "path" parameters)
   (puri:uri-path uri)
    "/")

This differs from RFC, https://tools.ietf.org/html/rfc6265#section-5.1.4, and practically, it breaks some applications (e.g., if cookie is set in /app/login, path is set to /app/login, not to /app, and rest of application does not receive it.)

Using (puri:uri-path (puri:merge-uris "." uri)) would help my use case, but I am not sure that this is in general exactly the same.

I can pull the one-line patch with this if wanted (docs and test cases as well?)