Fix a false positive for Rails/DynamicFindBy

[koic]

Fixes rubocop/rubocop-jp#54 (Ja)

This PR fixes a false positive for Rails/DynamicFindBy when not inheriting ApplicationRecord and without no receiver.

A dynamic matcher without receiver is only possible if it inherits ApplicationRecord (ActiveRecord::Base).

For example, Capybara's find_by_id and Token's find_by_valid_token should be accepted by default.

• https://www.rubydoc.info/github/jnicklas/capybara/Capybara/Node/Finders#find_by_id-instance_method
• https://github.com/fnando/tokens

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