Add tests for 1xx, 204 and 304 responses to response_test.rb

remove unnecessary send

`class_variable_get` is public from Ruby 1.9.
Ref: http://ruby-doc.org/core-2.2.0/Module.html#method-i-class_variable_get

Cleanup CHANGELOGs [ci skip]

Fix AJ tests on ruby 2.4 being caused since classes are unified for Integer

Fix AJ tests on ruby 2.4 being causes since classes are unified for Integer, and we create test name nased on arg class. Append ar as well to the test name

- Cleanup Active Record CHANGELOG.
- Add missing CHANGELOG for https://github.com/rails/rails/pull/25688.
- Clarify that assets requests logging is suppressed.

In response_test.rb, we haven't had a test to make sure that

1) these responses don't have a message-body as described in RFC7231[1]

2) 1xx and 204 responses must not have a Content-Length header field
   as described in RFC7230-section3.3.2[2]

[1] https://tools.ietf.org/html/rfc7231
[2] https://tools.ietf.org/html/rfc7230#section-3.3.2

Even though our implementation doesn't allow users to send
a Content-Length header field in a 304 response, sending the
header field is valid as mentioned in RFC7230-section3.3.2[2].
So I've decided not to test whether or not a 304 response has
the header.

The citation from the section is as follows;
```
A server MAY send a Content-Length header field in a 304 (Not
Modified) response to a conditional GET request (Section 4.1 of
[RFC7232]); a server MUST NOT send Content-Length in such a response
unless its field-value equals the decimal number of octets that would
have been sent in the payload body of a 200 (OK) response to the same
request.
```

fix to_param to maximize content

The documentation states that parameter values longer than 20 characters
will be truncated by words, but the example shows that a parameter based
on "David Heinemeier Hansson" (with id: 125) becomes "125-david" when
"David Heinemeier".length == 16 so why so short?

The answer lies in the use of the #truncate option omission: nil which
seems to have been intended to mean "nothing", but which actually causes
the default string "..." to be used. This causes #truncate to cleave
words until the "..." can be added and still remain within the requested
size of 20 characters.

The better option is omission: '' (which is probably what was originally
intended).

Furthermore, since the use of #parameterize will remove non-alphanumeric
characters, we can maximize the useful content of the output by calling
parameterize first and then giving truncate a separator: /-/ rather than
a space.

Remove unused boot_rails method and it's usage

Use the HOST environment variable for rails server #25677

[ci skip]

Fix race condition in websocket stream write

Add a changelog entry for #25692

[ci skip] Update documentation in AR SchemaStatements

This includes minor grammar fixes throughout the file.

No need to set `cookie_only` option from Rails

Fix typo in test case

Pathname might not be always initialized.

Require 'pathname' explicitly

Fix typo: caches_pages to caches_page. [ci skip]

Fix a small typo on doc: "caches_pages" -> "caches_page".

test with latest resque

Don't reap connections that have already been reassigned

updating doc for prefer require_relative [ci skip]

Correct defaults in documentation for ActionDispatch::SSL

`config.ssl_options` permits configuring various options for the middleware. Default options for HSTS (specified with the `:hsts` key in the options hash) are specified in `.default_hsts_options`. The documentation did not make clear these defaults, and in one case was wrong.

[ci skip] Update ConnectionPool documentation for readability purposes

[ci skip] Remove reference to Rails 4 in the initialization guide.

Following commit removed reference to rails 4, but there is one left.

https://github.com/rails/rails/commit/a9f50f87c38659d0e9425f86f613cf5328d55d27