Add 2 tests, 1 of which fails, to isolate the digest_caching behaviour causing #12521

If config.action_view.cache_template_loading = false, most likely in a development
configuration if config.cache_classes = false &
config.action_controller.perform_caching = true.

config.action_view.cache_template_loading defaults to the value of config.cache_classes
Ensure ActionView::Digest.cache is correctly cleaned up when
ActionView::Resolver.caching = false.
update changelog
update digestor code based on review
Removing unused fake models from actionpack tests
Merge pull request #12553 from tchandy/master

Removing unused fake models from actionpack tests
Fix code blocks identation on AR and AS CHANGELOGs
Merge pull request #12558 from lucasmazza/changelog-fixes

Fix code blocks identation on AR and AS CHANGELOGs
Rephrase ActiveRecord transaction rollback warning

[ci skip]
Merge pull request #12545 from nateberkopec/rephrase_callback_warning

Rephrase ActiveRecord transaction rollback warning [ci skip]
[ci skip] Fix number of methods added by association.

Also clean up some trailing spaces on line 1143-1146.
add a new local variable to track if digests are being stored, to ensure the cleanup works correctly
Fix typo in form_helper.rb
Merge pull request #12568 from adamyonk/patch-1

Fix typo in form_helper.rb
Merge pull request #12540 from wyaeld/bug/fix-recursive-digest

Ensure ActionView::Digestor.cache is correctly cleaned up
Ensure the state is clean after one failure
Merge pull request #12564 from JuanitoFatas/association-basics

[ci skip] Fix number of methods added by association.
Fixes #12423 - Additional select columns are removed when using includes