Modify detection of HTTPS

This modifies the detection of HTTPS by checking whether the lower-case value of the HTTPS-key of the server variable equals 'on'. Before that checked whether the value did *not* match 'off' which meant that the nginx-default for non-HTTPS connections (which, according to the [documentation](http://nginx.org/en/docs/http/ngx_http_core_module.html#var_https) is an empty string) did in fact match and returned that the connection *is* encrypted.

This fixes #362
# Conflicts:
#	src/functions/marshal_uri_from_sapi.php

src/functions/marshal_uri_from_sapi.php

@@ -171,7 +171,7 @@ function marshalUriFromSapi(array $server, array $headers)
     } else {
         $https = false;
     }
-    if (($https && 'off' !== strtolower($https))
+    if (($https && 'on' === strtolower($https))
         || strtolower($getHeaderFromArray('x-forwarded-proto', $headers, false)) === 'https'
     ) {
         $scheme = 'https';