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