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

Author: heiglandreas

src/functions/marshal_uri_from_sapi.php

@@ -168,7 +168,7 @@ function marshalUriFromSapi(array $server, array $headers) : Uri
             ));
         }
 
-        return 'off' !== strtolower($https);
+        return 'on' === strtolower($https);
     };
     if (array_key_exists('HTTPS', $server)) {
         $https = $marshalHttpsValue($server['HTTPS']);