Anbsence spec, with a column type other than String

Before `@attribute.to_s`:

    λ bundle exec rake appraisal spec spec/shoulda/matchers/active_model/validate_absence_of_matcher_spec.rb
    >> BUNDLE_GEMFILE=shoulda-matchers/gemfiles/4.0.0.gemfile bundle exec rake spec spec/shoulda/matchers/active_model/validate_absence_of_matcher_spec.rb

    973 examples, 4 failures, 56 pending

After `@attribute.to_s`:

    λ bundle exec rake appraisal spec spec/shoulda/matchers/active_model/validate_absence_of_matcher_spec.rb

    977 examples, 0 failures, 56 pending