Clean up how _changed_fields are set in BaseDocument._from_son

[wojcikstefan]

1. We were unnecessarily creating a changed_fields variable, assigning [] to it.
2. We were not modifying it at all.
3. In the end, we were setting obj._changed_fields to that variable instead of [] directly.

There's still more work to be done to make handling of _changed_fields simple and intuitive (see the TODO in the code), but this minor tweak will do for now.