[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.
I've amended the commit just adding the ` # :doc:` tag as suggested.