fix: let nested keys override non-mapping values in build_dict (#215) - #224

Problem

Fixes #215. A nested key that overrides a non-mapping value inside an existing mapping raises. With APP_DATABASE="{host: localhost, port: 5432}" and APP_DATABASE__PORT=5433, EnvSource("APP_", parse_yaml_docs=True) fails with AttributeError: 'int' object has no attribute 'update'. set_args({'database': {'port': 5432}, 'database.port': 5433}, dots=True) hits the same path. Confirmed on main HEAD 23bc86a.

Root cause

build_dict in confuse/util.py calls save_to[key].update(result) whenever key already exists, even if the existing value is a scalar, list or None.

Fix

Only call .update() when both the existing value and the new result are dicts. Otherwise the new, more specific value replaces the old one. Mapping-to-mapping shallow merges behave as before. One line changed in util.py, plus tests and a changelog entry.

How to test

$ pytest test/test_utils.py test/test_env.py

The three new tests fail on current main (AttributeError) and pass with this change. Full suite: 308 passed, 1 skipped (test_release.py, needs click).

Backward compatibility

No API changes. Inputs that used to raise now return the more specific value.

Assisted-by: Claude (code generation, reviewed and tested locally)