rapid-cdn: Avoid needless trailing slash sent to backend

Cover a case of not adding needless trailing / while accessing a backend from
top level of the URL.

As / are stripped while parsing the url, expose the situation that even if
url ends with a /, it won't be sent for top level access.

Here is the mapping of configured URL, path accessed on the CDN side and path
sent to the backend:

 * backend/index.html   frontend/     index.html
 * backend/index.html   frontend/a    index.html/a
 * backend/index.html   frontend/a/   index.html/a/
 * backend/index.html/  frontend/     index.html
 * backend/index.html/  frontend/a    index.html/a
 * backend/index.html/  frontend/a/   index.html/a/