remove HABTM special cases from the fixtures
remove HABTM special cases from associations classes
remove the HABTM preloader
remove HABTM special cases from reflections
remove HABTM associations
add a note to the changelog
Merge branch 'master' into habtm

* master:
  copy-edits the new guide with our maintenance policy [ci skip]
remove some duplicate code
add missing require
preheat habtm column cache
fix the variable name
`intiantiated` => `instantiated` [ci-skip]
Merge pull request #12430 from vipulnsward/changelog_fix

`intiantiated` => `instantiated` [ci skip]
Fix a typo in the code of Testing guide.

Replace the word «Accepts» by «Accept» in the example of custom request headers.
Merge pull request #12431 from GRoguelon/master

Fix a typo in the code of Testing guide. [ci skip]
Merge pull request #8791 from griffinmyers/master

Updated DirtyModel's @changed_attributes hash to be symbol/string agnostic

Conflicts:
	activemodel/CHANGELOG.md
map the association chain to table names
Port test from cf1904f to avoid future regression

Related issue: #11939, #12084
Merge pull request #12435 from sikachu/master-port-12084

Port test from cf1904f to avoid future regression
Fixed typo in documentation