Fix DecisionBankKiller.AlwaysKeepBanks property

The property AlwaysKeepBanks uses an update handler, which was not "used" in the constructor. This fixes it. See gaudi/Gaudi#16 for a wider mitigation of this easy-to-make bug. The buggy behaviour was that if you don't set the AlwaysKeepBanks from the options, it is effectively considered as an empty list rather than the default list of 3 elements.

I've increased the coverage of the test, which was previously not able to spot this bug.

While this code was in 2017-patches, it was never used in production.