Subscriptions prefs incorrectly set when saving

Actual behavior:

On a new list subscription (all preferences unset), when changing the digest mode from Regular to Mime Digests, all settings to the right of the digest mode get all changed to "No" (even though the Delivery status stays unset).

Expected behavior: all settings not being changed stay at their current value, even if this is "unset".

Note, my user and address preferences did have settings set to "Yes", which did not effect the actual behavior described here of everything being set to "No".

Issue #30 (closed) is related; my suggestion there is that we we want to keep unset ("inherit"?) as a valid choice for users and better explain in the UI the precedence order for the different preference tabs. This separate bug assumes retaining "unset" as a feature.