Don't error when `dup`ing a record with no PK

flash doesn't pass objects #15522 [ci skip]

Revert "Merge pull request #15394 from morgoth/fix-automatic-maintaining-test-schema-for-sql-format"

This reverts commit 46139d33, reversing
changes made to 8f247871.

Conflicts:
	activerecord/CHANGELOG.md

Add regression tests for keys on nested associations

This adds the regressions tests from issue #15893 to
master. It's checking that both strings and symbols are
accepted as keys for nested associations.

[ci skip] Fixed the documentation of xml_http_request? (aka xhr?)

Add configuration to enable mail previews

Silence warning emitted in tests

[ci skip] Add missing prologue section of upgrading ruby on rails guide.

The instance method `model_name` was being defined multiple times,
causing a redefinition warning.

Fix doc unwanted dl Admin:: [ci skip]

Adds `config.action_mailer.preview_enabled`

This allows mail previewing to be enabled easily in non-development
environments such as staging. The default is set to true for development
so no changes should be required to existing Rails applications.

The mail preview path can still be configured using the existing
`config.action_mailer.preview_path` configuration option.

Adding this avoids devs from having to do stuff like:
https://gist.github.com/lengarvey/fa2c9bd6cdbeba96526a

Update actionmailer/CHANGELOG with new configuration.
Update configuring guide with new configuratation.
Add `config.action_mailer.preview_path` to configuring guide.

Then we only need to extract host once.

Closes #15214

Since we are using both branches of the code is preferable to use
if/else over the early return.

Added String support for min and max properties for date field helpers

To avoid overload with database rollback

Added rollback method to ActiveModel::Dirty

Don't add a second method for attribute existence

We already had one in the public API that people can use more easily for
the transition

Fix 10.3 invite_email.body assert_match documentation example.

Currently there is no implicit conversion for Mail::Body to string
therefore .to_s must be applied. This is exampled in 10.2.2.

Update documentation on STI change handling

Documenting a potential source of confusion about how STI classes handle changes.

Simplify creation of default attributes on AR instance

Remove array workaround in PG quoting

Remove unused `array_member` from PG quoting for HStore columns

Hstore no longer needs additional quoting to be used in an array, the
array type handles it sufficiently.

We no longer need to do fancy legwork to make sure arrays use a type
object, now that schema methods use a real type object.