Toolbar icon incorrectly shown as paused when adblocking is active

(This is a copy of adblockinc/ext/adblock/adblock#285 (closed))

Steps to reproduce

1. Install ABP
2. Open options page
3. Add custom filter @@*$document,domain=~adblockplus.org
4. Navigate to: https://testpages.adblockplus.org/en/filters/blocking

Observed behavior

The toolbar icon is grey, as if the page was allowlisted.

Expected behavior

Toolbar icon should be red.