Explicitly define :_default_attribute_values using class_attribute in child class

[k-tsuchiya-jp]

Detail

Fix #100

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[k-tsuchiya-jp]

before

$ BUNDLE_GEMFILE=/home/zenet/default_value_for/gemfiles/rails_80.gemfile bundle exec rake
/home/k-tsuchiya-jp/.rbenv/versions/3.3.6/bin/ruby test.rb

Testing with ruby version 3.3.6

Testing with Active Record version 8.0.1

Testing with Action Pack version 8.0.1

Run options: --seed 21384

# Running:

...................................F..

Finished in 0.207456s, 183.1710 runs/s, 265.1159 assertions/s.

  1) Failure:
DefaultValuePluginTest#test_multiple_default_values_in_subclass_with_default_values_in_parent_class [test.rb:236]:
Expected: 5678
  Actual: nil

38 runs, 55 assertions, 1 failures, 0 errors, 0 skips
rake aborted!

after

$ BUNDLE_GEMFILE=/home/zenet/default_value_for/gemfiles/rails_80.gemfile bundle exec rake
/home/k-tsuchiya-jp/.rbenv/versions/3.3.6/bin/ruby test.rb

Testing with ruby version 3.3.6

Testing with Active Record version 8.0.1

Testing with Action Pack version 8.0.1

Run options: --seed 45765

# Running:

......................................

Finished in 0.200715s, 189.3235 runs/s, 279.0031 assertions/s.

38 runs, 56 assertions, 0 failures, 0 errors, 0 skips