Move types to the top level `ActiveRecord` namespace

`ActiveRecord::ConnectionAdapters::Type::Value` =>
`ActiveRecord::Type::Value`
Initial doc for TransactionIsolationError [ci skip]
Avoid hardcoded value in teardown.
Remove unused `Column#coder`

It appears this property was added, but never actually used. It would be
broken if it were, as it only type casts one way.
Unregister Mime::Type in teardown.
Merge pull request #15388 from zuhao/refactor_actionpack_send_file_test

Unregister Mime::Type in teardown.
Merge pull request #15387 from sgrif/sg-remove-column-coder

Remove unused `Column#coder`
Merge pull request #15386 from zuhao/refactor_actionpack_request_forgery_protection_test

Avoid hardcoded `request_forgery_protection_token` value in teardown.
Revert "Merge pull request #12778 from etehtsea/rake-default-fix"

This reverts commit ec0a2b57f67c9153cb5f7bbe2a3f66d13fe64bdd, reversing
changes made to a8bd7b16260c217290ef8fe3a166fcceac442c62.

This reverts commit 555ec36522011862c03b483c53be32410594a51e

This reverts commit 555ec36522011862c03b483c53be32410594a51e

By default, Rails uses the `default` Rake task to run all tests.
This commit changed the environment of the default task to `development`.
This clears the development database and has other negative consequences.
Merge pull request #15380 from Gaurav2728/gaurav-transaction_isolation_error

Initial doc for TransactionIsolationError [ci skip]
Formatting of content from b1db615 [ci skip]
Upgrading Ruby on Rails guide is no longer WIP [ci skip]
Merge pull request #15370 from sgrif/sg-type-namespace

Move types to the top level `ActiveRecord` namespace
Ensure custom properties work correctly with inheritance
Merge pull request #15389 from sgrif/sg-property-inheritance

Ensure custom properties work correctly with inheritance