Added rollback method to ActiveModel::Dirty
Don't add a second method for attribute existence

We already had one in the public API that people can use more easily for
the transition
Merge pull request #15991 from sgrif/sg-has-attribute

Don't add a second method for attribute existence
Merge pull request #14861 from igor04/dirty-rollback

Added rollback method to ActiveModel::Dirty
Add CHANGELOG entry for #14861 and document private methods on the API