Refactor max-age seconds

Remove duplicated `join_id_for`

Omit additional removing `config/cable.yml` 

Improve plugin generator codebase

Run Active Storage service tests in CI

Optimize routes indentation

Railties updates for frozen string literals.

Add document that you rake test single file by using DB(ex. postgresql)

Show minitest options in test runner help

Fix testing guide [skip ci]

Should escape regexp wildcard character `.`

Fix debugging rails application [ci skip]

Perform self-serialization once metadata is involved.

Clear mail after test

Fix CI failure due to reference type mismatch

`Firm.id` is a bigint if mysql2 adapter is used, but `firm_id` is an
integer. It will cause an out of range error.

https://travis-ci.org/rails/rails/jobs/264112814#L776
https://travis-ci.org/rails/rails/jobs/264112835#L919

If clear it before the test, the mail of the last executed test will not
be correctly cleared.

Therefore, executing the test with seed below will result in an error.

```
./bin/test -w --seed 55480
Run options: --seed 55480

# Running:

...........................................................................................................................................................F

Failure:
MailDeliveryTest#test_does_not_increment_the_deliveries_collection_on_error [/home/yaginuma/program/rails/master_y_yagi/rails/actionmailer/test/delivery_methods_test.rb:221]:
--- expected
+++ actual
@@ -1 +1 @@
-[]
+[#<Mail::Message:47011389364640, Multipart: false, Headers: <Date: Mon, 14 Aug 2017 07:48:40 +0900>, <From: test-sender@test.com>, <To: test-receiver@test.com>, <Message-ID: <5990d748ea5b2_29342ac1af8bcf40886f7@yaginuma.mail>>, <Subject: Test Subject>, <Mime-Version: 1.0>, <Content-Type: text/plain>, <Content-Transfer-Encoding: 7bit>>]

bin/test test/delivery_methods_test.rb:216
```

  `:force` is setted inside the method `generate_test_dummy`

Adds support for metadata even when using ActiveSupport::MessageEncryptor::NullSerializer.

Remove unnecessary Include parameter in rubocop.yml

`config/cable.yml` is never created if `--skip-action-cable` is given.

Use frozen string literal

The primary key on the owner record is abstracted as `join_foreign_key`.