Fixes for the server reachability test.

- Do not apply HTTPs redirection for challenge used by the test.
- Set the `User-Agent` to avoid 403 answer from site24x7.com.
- Handle JSON parsing failure of the received body.
- Better handling of different error cases.

docker/rootfs/etc/nginx/conf.d/include/force-ssl.conf

@@ -1,3 +1,10 @@
+set $test "";
 if ($scheme = "http") {
+	set $test "H";
+}
+if ($request_uri = /.well-known/acme-challenge/test-challenge) {
+	set $test "${test}T";
+}
+if ($test = H) {
 	return 301 https://$host$request_uri;
 }