Bugfixes

[BUGFIX] - Fixed an issue where the sort key field computation ignored certain model configurations; include_type_in_sort_key and type_is_primary_key
[BUGFIX] - Fixed an issue where sometimes unit tests involving freezegun would fail on deserialization
[BUGFIX] - Fixed an issue where during update operations, when one or more secondary index fields were changed, those changes were not propagated through to the computed index fields (e.g. gsi_sk).