[Enh] Made the ActiveModel::Dirty#clear_attribute_changes method public

In Rails 4.2 it is impossible to define a custom default value for a model's
attribute without making it appear as _changed?, especially when the model
is first initialized. Making this method public will allow such a behaviour,
without the need to use private APIs.
We should keep it with private visibility and just add a `# :doc:` comment after the `)` like we did with `changes_applied`