renew deprecated methods (#1444)

* renew deprecated methods

* replace "attribute_names" with the methods that exist in the Rails 5.2 and 6.0 APIs

lib/shoulda/matchers/active_model/helpers.rb

@@ -8,7 +8,7 @@ module Shoulda
         end
 
         def format_validation_errors(errors)
-          list_items = errors.keys.map do |attribute|
+          list_items = errors.to_hash.keys.map do |attribute|
             messages = errors[attribute]
             "* #{attribute}: #{messages}"
           end

spec/unit/shoulda/matchers/active_model/disallow_value_matcher_spec.rb

@@ -73,7 +73,7 @@ describe Shoulda::Matchers::ActiveModel::DisallowValueMatcher, type: :model do
 
         def custom_validation # rubocop:disable Lint/NestedMethodDefinition
           if self[:attr] != 'good value'
-            errors[:attr2] << 'some message'
+            errors.add :attr2, 'some message'
           end
         end
       end.new