UniqBeforePluck should recommend Distinct on rails 5

[ghiculescu]

In Rails 5, uniq was deprecated in favour of distinct on ActiveRecord relations: https://edgeguides.rubyonrails.org/5_0_release_notes.html#active-record-deprecations

This means the documentation here is wrong: https://github.com/rubocop-hq/rubocop-rails/blob/master/lib/rubocop/cop/rails/uniq_before_pluck.rb#L23

Specifically, Model.uniq.pluck(:id) will raise (NoMethodError (undefined method uniq' for #Class:0x000000000b339d10)). Whereas Model.distinct.pluck(:id)` will still work.

My suggestions are:

• Update docs to always recommend using distinct even if uniq was used beforehand.
• Update the error message to always recommend distinct no matter what method was matched.

Happy to do a PR if everyone is okay with this.