Push `valid_type?` up to abstract adapter

Remove duplicated private method in ActiveRecord::FinderMethods

Remove useless `select_values += select_values`

Use released arel

Follow up to ea9566f6

`select_values` is a local variable defined at previous line.
`select_values += select_values` is totally useless.

It's common in test cases at my job to have code like this:

    let(:today) { customer_start_date + 2.weeks }
    let(:earlier_date) { today - 5.days }

With this change, we can instead write

    let(:today) { 2.weeks.after(customer_start_date) }
    let(:earlier_date) { 5.days.before(today) }

Closes #27721

`valid_type?` should return true if a type exists in
`native_database_types` at least.

https://github.com/rails/rails/blob/v5.1.0.beta1/activerecord/lib/active_record/schema_dumper.rb#L136

Fix typo 'affect' -> 'effect' [ci skip]

Currently `change_column` cannot drop default if `null: false` is
specified at the same time. This change fixes the issue.

```ruby
  # cannot drop default
  change_column "tests", "contributor", :boolean, default: nil, null: false

  # we need the following workaround currently
  change_column "tests", "contributor", :boolean, null: false
  change_column "tests", "contributor", :boolean, default: nil
```

Closes #26582

AS CHANGELOG Pass 

Deprecate AbstractAdapter#verify! with arguments

Use ensure block for things we cleanup in tests

`DISTINCT` clause is applied inside aggregate function by
`operation_over_aggregate_column` if needed. Unneeded outside aggregate
function.

```ruby
  # Before
  author.unique_categorized_posts.count
  # => SELECT DISTINCT COUNT(DISTINCT "posts"."id") FROM "posts" INNER JOIN "categorizations" ON "posts"."id" = "categorizations"."post_id" WHERE "categorizations"."author_id" = ?  [["author_id", 2]]

  # After
  author.unique_categorized_posts.count
  # => SELECT COUNT(DISTINCT "posts"."id") FROM "posts" INNER JOIN "categorizations" ON "posts"."id" = "categorizations"."post_id" WHERE "categorizations"."author_id" = ?  [["author_id", 2]]
```

Closes #27615

In ca324a0d the flash middleware was effectively removed by its
constructor returning the app it was passed and the `commit_flash`
call was moved to the `ActionController::Metal#dispatch` method.
This broke any redirect routes that modified the flash because the
redirect happens before `dispatch` gets called.

To fix it, this commit adds a `commit_flash` call in the `serve`
method of `ActionDispatch::Routing::Redirect`.

Fixes #27992.

Improve docs for Rails::AppGenerator. [ci skip]

Add example so its easier to understand how one can overide
an app generator.

[close #24435] Send user_supplied_options to server

Closes #27429

In Ruby 2.4, BigDecimal(), as used by the Decimal cast, was changed so
that it will raise ArgumentError when passed an invalid string, in order
to be more consistent with Integer(), Float(), etc. The other numeric
types use ex. to_i and to_f.

Unfortunately, we can't simply change BigDecimal() to to_d. String#to_d
raises errors like BigDecimal(), unlike all the other to_* methods (this
should probably be filed as a ruby bug).

Instead, this simulates the existing behaviour and the behaviour of the
other to_* methods by finding a numeric string at the start of the
passed in value, and parsing that using BigDecimal().

See also
https://bugs.ruby-lang.org/issues/10286
https://github.com/ruby/bigdecimal/commit/3081a627cebdc1fc119425c7a9f009dbb6bec8e8

Soft-deprecate the `HashWithIndifferentAccess` constant

Allow ActiveSupport::MarshalWithAutoloading#load to take a Proc

Add missing gzip footer check in ActiveSupport::Gzip.decompress

This ensures that if we try to hard-deprecate it again in the future,
we won't break these behaviors.

Since using a `ActiveSupport::Deprecation::DeprecatedConstantProxy`
would prevent people from inheriting this class and extending it
from the `ActiveSupport::HashWithIndifferentAccess` one would break
the ancestors chain, that's the best option we have here.

A gzip file has a checksum and length for the decompressed data in its
footer which isn't checked by just calling Zlib::GzipReader#read.
Calling Zlib::GzipReader#close must be called after reading to the end
of the file causes this check to be done, which is done by
Zlib::GzipReader.wrap after its block is called.

Change `SystemTestCase.driven_by` to use `setup`/`teardown` hooks

Previously, `driven_by` would change the Capybara configuration when
the test case is loaded, and having multiple test classes with different
`driven_by` configs would fail as the last loaded would be effective.