Calling reset on a collection association should unload the assocation

Need to define #reset on CollectionProxy.
[ci skip] Added missing `file` delivery method
Merge pull request #13728 from semenyukdmitriy/master

Added missing `file` delivery method to the Configuration Guide
Remove duplicate merge
put core at the beginning so other classes can modify the behavior
Add failing test for ActiveModel::Errors#has_key? method

From the doc, this method should return false and not nil if there is no errors for this key
Merge pull request #13790 from kschlarman/collection_association_reset_fix

Calling reset on a collection association should unload the assocation
Make CollectionProxy#reset return self
Improve the CHANGELOG entry [ci skip]
Merge pull request #13791 from kei-s/remove_duplicate_merge

Remove duplicate merge
Move changed_attributes into dirty.rb
Move serialization dirty into serialization.rb
Fix ActiveModel::Errors#has_key? return value
Update ActiveModel::Errors.has_key? test
Merge pull request #13799 from kbrock/better_dirty

Better ActiveRecord hierarchy for Dirty and others
Merge pull request #13795 from bobbus/errors-has-key

Fix ActiveModel::Errors#has_key? returning nil
test boolean and number json param parsing
Merge pull request #13818 from arthurnn/small_test_case

test boolean and number json param parsing
unify param.require tests
Merge pull request #13819 from arthurnn/one_require_test

unify param.require tests
Remove unused assignment to fix warnings in enum test.