http to https detection not working

According to `man shellinaboxd`:

            shellinaboxd can distinguish between SSL/TLS requests and unencrypted requests. It also knows how to negotiate Server Name Identification, allowing the use of a single port for all types of requests even when virtual hosting.

This used to work on previous siab versions: run SIAB with -p 80, point a browser to http://yourhost , SIAB detected an http connection and would "upgrade" it to https. With 2.19, however, this is not working: https://yourhost:80 will work fine, but http://yourhost will result in this:

Resolving XXXX
Connecting to XXX (XXXXX)|X.X.X.X|:80... connected.
HTTP request sent, awaiting response... No data received.
Retrying.

This is being seen after an upgrade to xenial, package is:
shellinabox:amd64/xenial 2.19 uptodate