fix: ensure typed store attributes are cast even without default option

[glebson1988]

Fixes #49

What is the purpose of this pull request?

Fix incorrect type casting for store_attribute typed keys when default: is not provided.
Previously, values could remain uncasted (e.g. String instead of BigDecimal) after create/load in some class ownership/decorated scenarios.

What changes did you make? (overview)

• Updated store_attribute setup logic in lib/store_attribute/active_record/store.rb:
  • Removed the options.key?(:default) gate from the owner-change re-decoration path.
  • Now re-decoration happens whenever store owner changes, not only for attributes with defaults.
  • Updated owner assignment condition accordingly.
• Added regression specs in spec/cases/store_attribute_spec.rb:
  • typecasts inherited typed store attributes without defaults on reload
  • typecasts typed store attributes without defaults on reload

Is there anything you'd like reviewers to focus on?

Please focus on:

• correctness of the owner/re-decoration condition in store_attribute
• potential side effects for defaults behavior and STI/inheritance flows
• whether test coverage is sufficient for the reported regression path

Checklist

• I've added tests for this change
• I've added a Changelog entry
• I've updated a documentation (Readme)