bug: Thunderbird sometimes reuses previous image when settings have changed

Steps to Reproduce

1. Install the add-on.
2. Go to the add-on preferences.
3. Set the default avatar preference to anything but Disabled.
4. Update a contact (The contact will have the default avatar assigned to them.)
5. Go back to the preferences
6. Set the default avatar to a different value.
7. Go to the same contact modified in step 4, and remove the avatar.

This should use the new default avatar, but despite the network tab showing the correct avatar was fetched, Thunderbird assigned the previously downloaded avatar to the contact.