This basically reverts 8da30ad6

This basically reverts 618268b4

In order to keep this method compatible with the Ruby 2.5 version of Hash#slice.
This bahavior is actually slightly incompatibile with previous versions of Active Support
but it might not cause a real problem, since HWIA, the biggest use case of Hash subclassing here,
already overrides `slice` to return another HWIA.

since r60229

It should be initialized only when polymorphic associations.

"The dependency activerecord-jdbcsqlite3-adapter (>= 1.3.0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby, x64-mingw32, x86-mingw32 but the dependency is only for java. To add those platforms to the bundle, run `bundle lock --add-platform java`."

Allow symbol list for ignored_columns

Remove obsolete documentation for html_escape [ci skip]

Return correct exit status of ujs test

The `Process::Status#to_i` returns the bits in stat. If need exit status,
need to use `#exitstatus`.
Ref: https://ruby-doc.org/core-2.4.0/Process/Status.html#method-i-to_i

Instructions to use `h` or `html_escape` in ERB templates were added to 
`actionpack/lib/action_view/template_handlers/erb.rb` in a1b03493 (Rails 
2.1), but ERB has automatically escaped values since Rails 3.

`:api:` tag was removed in 5349f231 since RDoc doesn't support `:api:`
tag. But those methods are not private API, they are public API for
renderers. The renderers should be able to know that they can override
this method.

PhantomJS is abandoned, replace it with Selenium/Chrome headless

Fix callback in rails ujs

Remove unused `FixtureTemplate` class

`FixtureTemplate` is no longer used since 3d7892d4.

Fix `bin/rails db:setup` and `bin/rails db:test:prepare` create  wrong ar_internal_metadata's data for a test database.

Improve out-of-box experience with System tests including chromedriver-helper by default

Enable `hstore` extention disabled at the end of `test_migrate_enable_and_disable_extension`

Enable `hstore` extention disabled at the end of `InvertibleMigrationTest#test_migrate_enable_and_disable_extension`
to avoid failure of `PostgresqlArrayTest#test_schema_dump_with_shorthand`
which expects `hstore` extension enabled.

Set proper migration' version in bug report templates

Add mention how to delete several records in bulk to Active Record Basics Guides [ci skip]

Remove unused `UnknownController` class

`UnknownController` was added in b1999be5, but it is not used anywhere.

Remove unnecessary `allow_failure: true` option

`routes` task always returns zero to status, so status is not to non-zeno.
Ref: https://github.com/rails/rails/blob/b1867c480dd5476948ff0492ea2f91e2c2fcb04b/railties/lib/rails/tasks/routes.rake#L30

Fix typo in `allow_failure` argument [ci skip]

Ref: https://github.com/rails/rails/blob/3be123ba26cad461a80d7d680819e71c1388a241/railties/test/isolation/abstract_unit.rb#L243

42 compatibility - use int instead of bigint

Add headless chrome driver to System Tests