Fix Baseclass becomes! subclass.
Bring type casting behavior of hstore/json in line with serialized

`@raw_attributes` should not contain the type-cast, mutable version of
the value.
Use a frozen Set instance for CGI_VARIABLES.

Also expand the CGI_VARIABLE name listing to multiple lines for cleaner diffs and legibility.
Merge pull request #14971 from versioncontrol/#14785

Baseclass becomes! subclass
docs, update `includes` + `where` example in AR guide. [ci skip]

Closes #15541.
`bin/rake db:migrate:status` works with legacy migration numbers.

The migration numbers were normalized different ways. This left
the task output in an inconsistent state.

Closes #15538.
Merge pull request #15540 from nbibler/http-headers-set

Use a frozen Set instance for CGI_VARIABLES.
Merge pull request #15503 from sgrif/sg-json-hstore-storage

Bring type casting behavior of hstore/json in line with serialized
serialized Type should delegate `type_cast_for_write` to underlying Type

This adds a regression test for #14411, which was fixed by #15503.

Closes #14411
Closes #14595
test refactor, don't hardcode `primary_key_prefix_type` default.

/cc @zuhao
test refactor, don't hardcode default for `table_name_prefix`.
`ActiveRecord::SchemaMigration` has no primary key.

Before this patch, using `ActiveRecord::Base.primary_key_prefix_type`
with `:table_name_with_underscore` would change the `SchemaMigration` model
to have a primary key. This resulted in broken queries for PG because it tried
to return the inserted PK (which does not exist).

Closes #15051.
Closes #15419.
credit JoseLuis Torres for #15051. [JoseLuis Torres]

[ci skip]
Add missing test cases for schema dumping defaults
Merge pull request #15533 from sgrif/sg-missing-schema-tests

Add missing test cases for schema dumping defaults
Restore test deliveries properly in ActionMailer.

`ActionMailer::Base.delivery_method` and
`ActionMailer::Base.perform_deliveries` have leaked states.

"delivery method can be customized per instance" and "delivery method
can be customized in subclasses not changing the parent" in
delivery_methods_test.rb will fail if test_helper_test.rb (in which
TestHelperMailerTest is inherited from ActionMailer::TestCase) runs
before it.
Merge pull request #15544 from zuhao/refactor_actionmailer_test_case

Restore test deliveries properly in ActionMailer.