Since `ActionController:Parameters` does not inherit `Hash`, need to
explicitly convert it to `Hash`.
Also, `Parameters#to_h` returns `Hash` whose key is `String`. Therefore,
if merge as it is, the value will not be overwritten as expected.

[ci skip] Fix ParameterMissing exception name in docs

Fix `rake db:schema:load` with subdirectories

Should be `ActionController::ParameterMissing` and not
`ActionController::MissingParameter`.

Corresponding change was done in guides in
https://github.com/rails/rails/pull/9816.

Related #25174.

`db:schema:load` doesn't work with subdirectories like previous
`db:migrate:status`. `Migrator.migration_files` should be used in
`assume_migrated_upto_version` to fix the issue.

Fix `rake db:migrate:status` with subdirectories.

`db:migrate` supports subdirectories and have a test.

https://github.com/rails/rails/blob/v5.1.0.beta1/activerecord/test/cases/migrator_test.rb#L78-L85

But `db:migrate:status` doesn't work with subdirectories. It is due to
`Dir.foreach(path)` is not the same with `Dir["#{path}/**/[0-9]*_*.rb"]`.

I extracted `migration_files` and sharing it in the both to fix the
issue. And added tests for `db:migrate:status`.

Silence puma startup messages in system test

Don't share `options` with a reference type column

Fx system test example [ci skip]

Since test suffix is automatically granted, it is not necessary to
specify it in generator.
Also, updated the generated file to contents actually generated.

Fixes #28109

Sharing `options` causes some unexpected behavior. If `limit: 2` is
specified, this means that 2 bytes integer for a reference id column and
2 chars string for a reference type column. Another example, if
`unsigned: true` is specified, this means that unsigned integer for a
reference id column, but a invalid option for a reference type column.
So `options` should not be shared with a reference type column.

So calling `filename_with_extensions` omitting the second argument like this
https://github.com/slim-template/slim-rails/blob/8dbc1fbf859ebfa95b0884a0196a6ad9f0ca9cd5/lib/generators/slim/scaffold/scaffold_generator.rb#L10
causes NoMethodError.

fixes #28275

passing unique parameter to belongs_to wrongly

Various refactoring to association scopes and joins

Reflections only use their own information to create a `join_keys`
object.  This means that we can call `join_keys` on a reflection object
and have it be context-free.

Add iso8601 and rfc3339 parsing to timezones

For naming consistency when using the RFC 3339 profile
of ISO 8601 in applications.

The `Time.xmlschema` and consequently its alias `iso8601` accepts
timestamps without a offset in contravention of the RFC 3339
standard. This method enforces that constraint and raises an
`ArgumentError` if it doesn't.

Previously there was no way to get a RFC 3339 timestamp
into a specific timezone without either using `parse` or
chaining methods. The new method allows parsing directly
into the timezone, e.g:

    >> Time.zone = "Hawaii"
    => "Hawaii"
    >> Time.zone.rfc3339("1999-12-31T14:00:00Z")
    => Fri, 31 Dec 1999 14:00:00 HST -10:00

This new method has stricter semantics than the current
`parse` method and will raise an `ArgumentError`
instead of returning nil, e.g:

    >> Time.zone = "Hawaii"
    => "Hawaii"
    >> Time.zone.rfc3339("foobar")
    ArgumentError: invalid date
    >> Time.zone.parse("foobar")
    => nil

It will also raise an `ArgumentError` when either the
time or offset components are missing, e.g:

    >> Time.zone = "Hawaii"
    => "Hawaii"
    >> Time.zone.rfc3339("1999-12-31")
    ArgumentError: invalid date
    >> Time.zone.rfc3339("1999-12-31T14:00:00")
    ArgumentError: invalid date

Previously there was no way to get a ISO 8601 timestamp into a specific
timezone without either using `parse` or chaining methods. The new method
allows parsing directly into the timezone, e.g:

    >> Time.zone = "Hawaii"
    => "Hawaii"
    >> Time.zone.iso8601("1999-12-31T14:00:00Z")
    => Fri, 31 Dec 1999 14:00:00 HST -10:00

If the timestamp is a ISO 8601 date (YYYY-MM-DD) then the time is set
to midnight, e.g:

    >> Time.zone = "Hawaii"
    => "Hawaii"
    >> Time.zone.iso8601("1999-12-31")
    => Fri, 31 Dec 1999 00:00:00 HST -10:00

This new method has stricter semantics than the current `parse` method
and will raise an `ArgumentError` instead of returning nil, e.g:

    >> Time.zone = "Hawaii"
    => "Hawaii"
    >> Time.zone.iso8601("foobar")
    ArgumentError: invalid date
    >> Time.zone.parse("foobar")
    => nil

Fix spelling in docs

[ci skip]

It turns out that we don't need to require system tests in the railties
test helper so we can remove it. If you're using system tests they will
be loaded by inheriting from ActionDispatch::SystemTestCase and the
routes will be loaded by ActionDispatch::IntegrationTest.

Scopes can only ever be *not* reflection objects when they are passed in
to the Reflection constructor.  Given this fact, we can eliminate is_a
checks and an intermediate array object by just asking the reflection
object for join scopes.

Remove `node` parameter to `join_constraints`

I don't think we actually need this parameter anymore.  Nobody seems to
be using it.

passing `unique` parameter to belongs_to not right,