[Bugfix] Allow to use frozen scope for Rails/UniqueValidationWithoutIndex

[krim]

AST for a frozen object looks like:

=> s(:send,
  s(:array,
    s(:sym, :b_id),
    s(:sym, :c_id)), :freeze)

The type for that is send, so it fails to detect the correct type for a scope.

[koic]

This looks good to me. Can you add the changelog entry?
https://github.com/rubocop-hq/rubocop-rails/blob/v2.7.1/CONTRIBUTING.md#changelog-entry-format

[krim]

Yep, done

[koic]

Thanks!