Fix duplicable? for Complex and Rational

quiet generators log in test

Remove warning in ActiveJob

This quiet the following log.

```
   create  app/mailers/notifier_mailer.rb
   invoke  erb
   create    app/views/notifier_mailer
identical    app/views/layouts/mailer.text.erb
identical    app/views/layouts/mailer.html.erb
   create    app/views/notifier_mailer/foo.text.erb
   create    app/views/notifier_mailer/foo.html.erb
   invoke  test_unit
   create    test/mailers/notifier_mailer_test.rb
   create    test/mailers/previews/notifier_mailer_preview.rb
```

Currently it causes:

```
activejob/Rakefile:5: warning: already initialized constant ACTIVEJOB_ADAPTERS
```

Document and update API for `skip_parameter_encoding`

This commit changes `parameter_encoding` to `skip_parameter_encoding`.
`skip_parameter_encoding` will set encoding on all parameters to
ASCII-8BIT for a given action on a particular controller.  This allows
the controller to handle data when the encoding of that data is unknown,
for example file systems or truly binary parameters.

See [this test](https://gist.github.com/utilum/78918f1b64f8b61ee732cb266db7c43a).

fix fixtures loading for models with same table names in different databases

Updated the docmentation because "options supports" is not grammatically
correct.

Add all the available options, and correct the samples of the
`javascript_include_tag`.

Ruby 2.4 compatibility

Very similar to PR #25758, see more in depth reasoning there.

Bumped to Ruby 2.4 compatible versions

Rewrite bits and pieces to mention the command infrastructure.

Revise the "XML is not HTML" test

Only default the response charset when it is first set

It was depending on a side-effect of the old html-scanner, so was no
longer proving what it intended to. Instead, assert more directly about
the resulting observable difference.

If it is explicitly cleared (e.g., response.sending_file = true), then
we should not try to set it again.

Do not clear HTTP_COOKIES header after request

make work bin/test scripts with line filter

Fix typo in channel generator usage message

Add note on defaults block option

`ARGV.shift` before calling Rails generators

Remove useless `test_supports_primary_key`