Added partial days support to `DateTime`'s `advance` method.

You can now add partial days (e.g. 2.5.days) to `DateTime` with the advance method.
This was acheived by mimicing the `advance` implementation in `Time`.
Fixed changelog
WIP: pass existing joins to construct_join_dependency
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
Skip if the association was already loaded.
Remove unnecessary flatten! method call.
[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.
Call get_routes_as_head only on HEAD requests.
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.
Merge pull request #15324 from andriytyurnikov/simple_rake_stats_for_everything

simple solution that brings rake stats task to all rails project
Merge pull request #15316 from akshay-vishnoi/test-cases

Fix test cases for inflector.rb
Merge pull request #15325 from tgxworld/no_need_to_call_to_s

Remove duplicated to_s method call.