[Enh] Changed the visibility of the ActiveModel::Dirty#clear_attribute_changes method

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 publicly visible will allow such a behaviour,
without the need to use private APIs.
Merge pull request #17545 from marzapower/dirty_public_api

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