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
unwrap the constraints object on initialization, eliminate loops

Unwrap Constraints objects.  I don't actually think it's possible
to pass a Constraints object to this constructor, but there were
multiple places that kept testing children of this object.  I
*think* they were just being defensive, but I have no idea.
Remove unnecessary require of Minitest.

Minitest has already been required when calling Minitest.autorun.
Merge pull request #15305 from tgxworld/remove_unnecessary_require

Remove unnecessary require of Minitest.
Remove unnecessary flatten! method call.
The correct status to test should be :switching_protocols.
Convert source to string if it is present.
Merge pull request #15311 from zuhao/refactor_actionpack_bare_metal_test

The correct status to test should be :switching_protocols.
Merge pull request #15312 from JuanitoFatas/action_view/asset_path

Convert source to string if it is present.
[ci skip] Update GFM link in guides guidelines.

Also remove a duplicated sentence.
Merge pull request #15315 from JuanitoFatas/guides-guidelines

[ci skip] Update GFM link in guides guidelines.
Fix test cases for inflector.rb
`yield` was missing in #with_dup, due to which 57 assertions were not running
in real, and all tests were passing by default.
Error was introduced in commit - 31ceb5e.
Rename Posts to Articles in Guides' Getting Started App, continuation of #15215 [ci skip]
simple solution that brings rake stats task to every kind of rails project
Merge pull request #15310 from tgxworld/small_refactor_get_routes_as_head

Remove unnecessary flatten! method call.
Remove duplicated to_s method call.
remove unused variable warning
`warning: assigned but unused variable - album`
Fix docs for ActionController::Renderers.add
Merge pull request #15328 from kuldeepaggarwal/remove-warnings

remove unused variable warning.