lua: properly handle corner cases in changes

lua: properly handle corner cases in changes
The previous rework of the change serialization caused some deltas to incorrectly
get promoted to list appends. Rework the code to explicitely check for an add list
change type, in all other cases simply overwrite existing fields with the current
string value instead of coercing existing values into a table and append to that.
This hopefully sorts out the remaining issues.