Fix errors that occur when unrelated tag is investigated by Rails/ContentTag

[r7kamura]

I encountered a strange error when running rubocop-rails on our Rails app that uses FactoryBot.

The file that caused the problem was, in simplified form, the following code:

FactoryBot.define do
  factory :article do
    tag factory: :article_tag
  end
end

and the error output was like this:

$ bundle exec rubocop spec/factories/articles.rb 
Inspecting 1 file
An error occurred while Rails/ContentTag cop was inspecting /workspace/spec/factories/articles.rb:3:4.
To see the complete backtrace run rubocop -d.
.

1 file inspected, no offenses detected

1 error occurred:
An error occurred while Rails/ContentTag cop was inspecting /workspace/spec/factories/articles.rb:3:4.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
https://github.com/rubocop/rubocop/issues

Mention the following information in the issue report:
1.36.0 (using Parser 3.1.2.1, rubocop-ast 1.21.0, running on ruby 2.7.2) [x86_64-linux]

This Pull Request has been created to fix this problem.

We could add {spec,test}/factories/**/*.rb to Exclude section of this cop, but that would not prevent the problem from reoccuring in other places. Since extensive investigation is performed on the generic method name tag, I think it should be checked more rigorously in this way.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

Nice! Can you add a changelog entry?