Connections in reader mode are not FPI

For example, this can be seen in http://expyuzz4wqqyqhjn.onion/index.html.

The top level html is fetched via the catch-all circuit (--unknown--):

Torbutton INFO: tor SOCKS: http://expyuzz4wqqyqhjn.onion/index.html via
                       --unknown--:cc55771ba6d6e8c6542066863a47e223

While the rest of resources are fetched via about.ef2a7dd5-93bc-417f-a698-142c3116864f.mozilla socks username

Torbutton INFO: tor SOCKS: http://expyuzz4wqqyqhjn.onion/static/images/home/png/block-trackers@3x.png?h=70991bcb via
                       about.ef2a7dd5-93bc-417f-a698-142c3116864f.mozilla:bde927d14c46a03e9a1501261b9798ba

which is mozilla's ABOUT_URI_FIRST_PARTY_DOMAIN constant.