domain isolator doesn't work for ftp channels

Here's my fix: https://github.com/arthuredelstein/torbutton/commit/0116a2ac9597a20a0e9bb8910e8a1f5e477d4c32 You can test it by setting torbutton's loglevel to 3 and browsing to https://arthuredelstein.github.io/tordemos/ftp-image-in-html.html