:tada::tada::tada:

It makes sense to be as strict as possible
with headers from the outside world,
but allowing @ to support Apache's mod_unique_id
(see #31644) seems OK to me

Fixes #31823.

[ci skip]

### Summary

This PR changes .rubocop.yml.

Regarding the code using `if ... else ... end`, I think the coding style
that Rails expects is as follows.

```ruby
var = if cond
  a
else
  b
end
```

However, the current .rubocop.yml setting does not offense for the
following code.

```ruby
var = if cond
        a
      else
        b
      end
```

I think that the above code expects offense to be warned.
Moreover, the layout by autocorrect is unnatural.

```ruby
var = if cond
  a
      else
        b
      end
```

This PR adds a setting to .rubocop.yml to make an offense warning and
autocorrect as expected by the coding style.
And this change also fixes `case ... when ... end` together.

Also this PR itself is an example that arranges the layout using
`rubocop -a`.

### Other Information

Autocorrect of `Lint/EndAlignment` cop is `false` by default.
https://github.com/bbatsov/rubocop/blob/v0.51.0/config/default.yml#L1443

This PR changes this value to `true`.

Also this PR has changed it together as it is necessary to enable
`Layout/ElseAlignment` cop to make this behavior.

[ci skip]

These assertions did matter due to the inconsistent behavior of
[the #parameters method][1]. Today, it behaves consistently and they
could be removed. Also, one of the methods was stubbed somewhat
incorrectly, so it is better not to stub and instead, make them close
to more realistic use cases.

[1]: https://github.com/rails/rails/pull/13999#issuecomment-34601746

When a user tries to create a new attachment or blog and the matching table is missing from the database
(`active_storage_attachments` and `active_storage_blobs` by default), an informative error is displayed
that invites users to run the `active_storage:install` task.

`make_tmpname` was removed by https://github.com/ruby/ruby/commit/25d56ea7b7b52dc81af30c92a9a0e2d2dab6ff27.
In this case, we want a file name, not a `File`. So cannot use `Tempfile`.

Fixes #31458

`BigDecimal.new` has been deprecated in BigDecimal 1.3.3
 which will be a default for Ruby 2.5.

Refer
https://github.com/ruby/bigdecimal/commit/533737338db915b00dc7168c3602e4b462b23503

* This commit has been made as follows:

```
cd rails
git grep -l BigDecimal.new | grep -v guides/source/5_0_release_notes.md | grep -v activesupport/test/xml_mini_test.rb | xargs sed -i -e "s/BigDecimal.new/BigDecimal/g"
```
- `activesupport/test/xml_mini_test.rb`
Editmanually to remove `.new` and `::`

- guides/source/5_0_release_notes.md
This is a Rails 5.0 release notes.

Follow up of #31432.

and defaults to `Digest::MD5`.

Replaced calls to `::Digest::MD5.hexdigest` with calls to `ActiveSupport::Digest.hexdigest`.

Follow up of #31390.

Change the system tests to set Puma as default server only when the user haven't specified manually another server.

Matches Hash#each behaviour as used in Rails 4.

Follow up of 3c442b6d

Without this require, it will fail when run CSP test alone.
Ref: https://travis-ci.org/rails/rails/jobs/311715758#L2976

Use Object#deep_dup to safely duplicate policy values

Since this changes a default setting a changelog entry is important.

Not everyone uses iTerm2 and whereas Terminal.app on a mac just ignores
that and outputs the path, other terminals like those on Ubuntu do not.
A friendlier default is one that works by default.

Closes #31159
Closes #30957