fix: normalize attribute keys to symbols

[bharget]

Summary

Fixes a bug where passing attributes with string keys (e.g., from Rails params) would not properly override defaults.

Problem

The defaults hash uses symbol keys, but attributes.key?(key) failed when attributes had string keys:

# Before fix - broken with string keys
Service.new("name" => "John").computed  # callable saw default, not "John"

Solution

Normalize attribute keys to symbols at the start of initialize:

attributes = attributes.transform_keys(&:to_sym)

Tests

Added specs for string key handling with both static and callable defaults.