A tiny grammatical fix

[ci skip]
Fixes problem with replacing has_one association record with itself
Merge pull request #12834 from dredozubov/has_one_association_replacement

Fixes problem with replacing has_one association record with itself
Load test fixtures where data are needed

Without this, some tests here were not actually testing anything.
Avoid sorting an Array including objects from different Classes

addresses "ArgumentError: comparison of VerySpecialComment with SpecialComment failed" in ActiveRecord::DelegationRelationTest#test_#sort!_delegation_is_deprecated
tidy Active Record CHANGELOG. [ci skip].
Fixes #11079, we should not decrement counter_cache if self-referential association destroyed using :destroy method which has own counter_cache callbacks