List assert_{,no_}changes in the testing guide

Remove a redundant test assertion

This lists the `assert_changes` and `assert_no_changes` methods in the
guides. [ci skip]

Merge pull request #29208 from kamipo/default_env_fall_back_to_default_env_when_rails_env_or_rack_env_is_empty_string

`DEFAULT_ENV` falls back to `default_env` when `RAILS_ENV` or `RACK_ENV` is an empty string

Deprecate passing arguments and block at the same time to `count` and `sum` in `ActiveRecord::Calculations`

Deprecate passing arguments and block at the same time to `count` and `sum` in `ActiveRecord::Calculations`

`select`, `count`, and `sum` in `Relation` are also `Enumerable` method
that can be passed block. `select` with block already doesn't take
arguments since 4fc3366d. This is follow up of that.

Remove a redundant test case of command_recorder_test

Remove mentions of deprecated callbacks on ActionDispatch::Callbacks

Correctly set user_supplied_options when there is no whitespace in option specification

Fix server-generated JS response processing on IE9

Remove returning true in internal callbacks

AEAD encrypted cookies and sessions

ActionDispatch::Callbacks.to_prepare was removed in #27587
[ci skip]

Prevent extra `current_database` query for `encoding`/`collation`/`ctype`

Refactor enum to use `value` instead of `label` in the scope

Delegate `ast` and `locked` to `arel` explicitly

Fix pluralization of uncountables when given a locale

Enable extending even if scope returns nil

Remove unused `Association#interpolate`

Bring back delayed_job to test list

`delayed_job_active_record` 4.1.2 supports Rails 5.1.
Ref: https://github.com/collectiveidea/delayed_job_active_record/pull/138

If users added an attribute or otherwise changed a CurrentAttributes subclass
they'd see exceptions on the next page load.

Because `ActiveSupport::CurrentAttributes.current_instances` would keep
references to the old instances from the previous request.

We can fix this by clearing out the `current_attributes` before we unload
constants. Then any change to the model can be autoloaded again since its
slot isn't taken by an old instance.

We'll still have to call reset before we clear so external collaborators,
like Time.zone, won't linger with their current value throughout other code.

The app is booted by then, so there's no need to stash the code away in
some other script.

Since we're generating a key through the class name we can combine
the two Thread.current calls into a single hash version.

Add lazy loading to #keys and #values methods in Session

Fix regression in Numericality validator

Includes namespace in system test skelton when namespace is specified

Fixes #29247

Using `Association#interpolate` was removed since #11251.

a user input for a decimal column were ignored by numerically validations

`find` raises when it can't find a record, so we'll never reach the
else. Switch to `find_by` which returns nil when no record can be
found.

[ci skip]fix wrong variable name in docs

[ci skip]fix wrong variable name in docs