Improve Active Model Dirty API.

These methods may cause confusion with the `reset_changes` that
behaves differently
of them.

Also rename undo_changes to restore_changes to match this new set of
methods.

This method name is causing confusion with the `reset_#{attribute}`
methods. While `reset_name` set the value of the name attribute for the
previous value the `reset_changes` only discard the changes and previous
changes.

Add Rails::Application#config_for

This is a convenience for loading configuration for the current Rails
environment.

Conflicts:
	activerecord/CHANGELOG.md

Also updated the test case to reflect that

Closes #15936

Fixed JSON coder when loading NULL from DB

This reverts commit 6f3c64ee.

Conflicts:
	activerecord/CHANGELOG.md

If a counter_cache exists, use it for #empty?

Add ability to extend `rails server` command options parser

Log digest as :debug instead of :info

Fix empty host for an asset url when asset_host proc returns nil

Return an absolute instead of relative path from an asset url in the case of the `asset_host` proc returning nil

[ci skip] Fix grammar of eager loading for ActiveRecord::Associations

Refactor reflections from sub classes to delegates

Stash original path in `ShowExceptions` middleware

Address ORA-00972: identifier is too long when tested with Oracle

by using shorter attribute names.

`ActionDispatch::ShowExceptions` overwrites `PATH_INFO` with the status code
for the exception defined in `ExceptionWrapper`, so the path the user was
visiting when an exception occurred was not previously available to any custom
exceptions_app.

The original `PATH_INFO` is now stashed in
`env["action_dispatch.original_path"]`.

Dump PostgreSQL primary key with custom function as a default.

[ci skip] Improve documentation consistency for ActiveModel::Errors

*   Leave two blank lines between sections/packages for readibility.

*   Always indent bullet points with four spaces. This allows multi-paragraph
    points to align properly (see #16103), and it is the same as the format we
    already use in CHANGELOGs. Doing this consistently sets an easy precedent
    for future contributors to follow.

*   Always put the PR/commit link on its own line in the source. If the bullet
    point spans multiple paragraphs, put it on its own paragraph at the end.

[Godfrey Chan & Juanito Fatas]

[ci skip] Fix typos like `a html` to `an html` and 'an mail' to 'an email'.

Fix tests broken in #16149

Refactor Hash#transform_values and Hash#transform_values!