Explain that default_scope also influences update_all

This was not explicitly stated before and I needed to try it out to be
certain. A little explicit statement in the API docs might help here.
Remove is_a? check when ignoring tables

Technically changes the API, as it will allow any object which responds
to `===`. Personally, I think this is more flexible.
Fix includes on association with a scope containing joins along with conditions
on the joined assoiciation
Creates an ApplicationMailer and layout by default, including html and body tags to reduce spam score
Move test that depends on ActiveRecord

This commit moves a test from `test/template` to `test/activerecord` since the
test depends on ActiveRecord.

This matches the documentation from [RUNNING_UNIT_TESTS](https://github.com/rails/rails/blob/f28d1ddd507174ac233b773cc4f35c3c05ad32e7/actionview/RUNNING_UNIT_TESTS.rdoc):

> Test cases in the test/activerecord/ directory depend on having activerecord and sqlite3 installed.
> If Active Record is not in actionview/../activerecord directory, or the sqlite3 rubygem is not installed, these tests are skipped.
> Other tests are runnable from a fresh copy of actionview without any configuration.

---

More details about this commit.

All the tests starting with `require 'active_record_unit'` are already
inside `test/activerecord`, except for the one test this commit moves.

If you don't have `active_record` on your machine, the following command
currently fails on master:

```bash
```
[ci skip] correct example output and filename in pg guide
Merge pull request #17712 from y-yagi/correct_postgres_guide

[ci skip] correct example output and filename in pg guide
Fixing wrong link in 'Ruby on Rails Security Guide' [ci skip]

The URL

http://www.h-online.com/security/Symantec-reports-first-active-attack-on-a-DSL-router--/news/102352

points to an article titled 'The H is closing down'. The good one is:

http://www.h-online.com/security/news/item/Symantec-reports-first-active-attack-on-a-DSL-router-735883.html
Merge pull request #17716 from javiervidal/fixing_documentation

Fixing wrong link in 'Ruby on Rails Security Guide' [ci skip]
Remove outdated comments [ci skip]

They were introduced in 23b6def; the serial stuff has been removed since a5d80f8
Rename the primary key index when renaming a table in pg

Also checked to make sure this does not affect foreign key constraints.
(It doesn't).

Fixes #12856
Closes #14088
Fix build failures

For some reason changing `.find` to `.unscoped.find` in
https://github.com/rails/rails/commit/f766abd4cf3eb75469d3646cfb6d85e668c619f3
caused `scoping` to leak in some tests when run in isolation (looks like
a concurrency issue?). `relation_scoping_test.rb` is a case that failed.
From what I can tell it should not be possible, but changing to the
block form fixes it. There is a deeper issue that I can't seem to find.
/cc @senny
Anchor should not be appended when set to nil/false.

Fixes https://github.com/rails/rails/issues/17714.
Add changelog entry for .reflections API change

`.reflections` public API changed to return a String instead of a Symbol
as keys.

see commit 1f31488499111fdfce79d8dc1cc8fb008f7cdb25 and 6259e4e2dcca9a79f22f96658c33efe81936bc0d

[fixes #16928]
[fixes #17610]
Update reflections public API doc
Merge pull request #17718 from arthurnn/fixes_16928

Fixes 16928, improve reflections public API doc [ci skip]
Fix explanation for ActionDispatch::Callbacks.

ActionDispatch::Callbacks dose not run the prepare callbacks,
so change with comment on ActionDispatch::Callbacks.
Merge pull request #17719 from yui-knk/fix/rails_on_rack_guide

Fix explanation for ActionDispatch::Callbacks. [ci skip]
[ci skip] Dispatching is a verb :bow:

cc #17719
[ci skip] Fix class name of middleware.