adding X-Forwarded-Proto support in is_ssl()

This commit adds support for X-Forwarded-Proto in case dokuwiki runs
behind a reverse (SSL) proxy, but the connection between the proxy and
the web server running dokuwki is plain HTTP. If an authenticated
user is accidentaly dropped to HTTP from HTTPS the securecookie setting
should be honored and the redirection when saving or canceling a page
edit should point to the correct protocol. This happens every time a
user hits cancel after editing a page, and likely even after saving. If
the webserver doesn't enforce HTTPS the coockie might be sent in clear.
Adding unit test for is_ssl()

this should test the correctness of the return value of is_ssl()
function based on various combinations of PHP environment variables