Cleaning up ActiveModel::Dirty tests

* Clarifying what the #changed method returns
* Adding tests to describe what the #changed_attributes returns
* Updating test name based on pull request comment
* Moving the test lower in the file per pull request comment
Update guides/source/active_record_validations.md

Duplicated `update`
use case statement
allow :dirs option for .enumerate

Allows custom rake tasks to be defined using:

  SourceAnnotationExtractor.enumerate 'TODO|FIXME', dirs: %w(app lib), tag: true
Merge pull request #8940 from adomokos/adding_tests_for_changed_attributes

Cleaning up ActiveModel::Dirty tests
Merge pull request #8954 from davetoxa/patch-1

Update guides/source/active_record_validations.md
Remove header bloat introduced by BestStandardsSupport middleware

The same headers were being duplicated on every request.
strong parameters exception handling
Merge pull request #8914 from nilbus/fix-header-bloat

Remove header bloat introduced by BestStandardsSupport middleware
Merge pull request #8958 from balexand/strong_parameters_exception_handling

Strong parameters exception handling
Merge pull request #8955 from burns/source_annotation

Allow a `:dirs` option to `SourceAnnotationExtractor.enumerate`