Latest commit 7815039 Dec 4, 2017 @karmi karmi [MODEL] Fixed the handling of changed attributes in `Indexing` to wor…
…k with older Rails versions

This patch builds on work in #738 by @jkeam and #758 by @Geesu to prevent deprecation warnings on Rails 5.1
due to changes in the handling of "changed attributes", originally reported by @davedash in #714.

It's primary focus is to restore the compatibility with older Rails versions (so we don't break compatibility
without proper version bump and in a single isolated commit), and to make the naming a bit more descriptive.

First, the condition has been changed to work with the `changes_to_save` and `changes` methods, as opposed
to the original `changed_attributes` / `attributes_in_database` naming. This communicates much more clearly
the intent, and the original usage of `changed_attributes` has been misleading.

Also, the "internal instance variable" which keeps track of the changes has been renamed to `@__changed_model_attributes`,
in order to cleary differentiate it from the `changed_attributes` method name in older ActiveRecord versions.

Closes #758