caddy-frontend: https-url is correctly implemented

As test_https_url proves, the separation of url and https-url is correctly
implemented.

software/caddy-frontend/TODO.rst

 Generally things to be done with ``caddy-frontend``:
 
- * implement back ``url`` and ``https-url`` on slave, that if ``https-url`` is defined it is used on HTTPS enabled part, and ``url`` is used on HTTP part
  * tests: add assertion with results of promises in etc/promise for each partition
  * check the whole frontend slave snippet with ``caddy -validate`` during buildout run, and reject if does not pass validation
  * ``apache-ca-certificate`` shall be merged with ``apache-certificate``