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
Added documentation for the :param option for resourceful routing
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.
Always construct route objects with Constraint objects
push is_a?(Dispatcher) check in to one place
eliminate dispatcher is_a checks
Constraints#app should never return another Constraints object, so switch to if statement
Call get_routes_as_head only on HEAD requests.
one fewer is_a check
nothing is passed to `rack_app` anymore, so rm the params
pass a request to `matches?` so we can avoid creating excess requests
pass the request object to the application
Rename Posts to Articles in Guides' Getting Started App, continuation of #15215 [ci skip]
remove unwanted `to_sym` call.
correct doc for (audio|video)_tag [ci skip]
Escape user input before showing in the page.

This is not a security issue since this page is not present in
production and user have to type something in the field but is better to
escape the input.