Merge pull request #15210 from arthurnn/fix_hbtm_reflection

Fix habtm reflection
Conflicts:
    activerecord/CHANGELOG.md
    activerecord/lib/active_record/counter_cache.rb
    activerecord/lib/active_record/reflection.rb
    activerecord/test/cases/reflection_test.rb
Use .to_s on _reflections lookup
Refactoring .reflections public method.

Now the internal reflections will hold a reference to its public
representation, so when the outside world calls `Account.reflection` we
can build a list of public reflections.