These requires were added only to change deprecation message

This patch uniformizes warning messages. I used the most common style
already present in the code base:

* Capitalize the first word.

* End the message with a full stop.

* "Rails 5" instead of "Rails 5.0".

* Backticks for method names and inline code.

Also, converted a few long strings into the new heredoc convention.

The current style for warning messages without newlines uses
concatenation of string literals with manual trailing spaces
where needed.

Heredocs have better readability, and with `squish` we can still
produce a single line.

This is a similar use case to the one that motivated defining
`strip_heredoc`, heredocs are super clean.

- Also one minor change for documenting url_for method in ActionController::Metal.
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.. even when the producer is blocked for a write.

avoid freezing the headers until the web server has actually read data
from the body proxy.  Once the webserver has read data, then we should
throw an error if someone tries to set a header

when streaming responses, we need to make sure the cookie jar is written
to the headers before returning up the stack. This commit introduces a
new method on the response object that writes the cookie jar to the
headers as the response is committed.  The middleware and test framework
will not write the cookie headers if the response has already been
committed.

fixes #14352

`render :body` should just not set the `Content-Type` header. By
removing the header, it breaks the compatibility with other parts.

After this commit, `render :body` will returns `text/html` content type,
sets by default from `ActionDispatch::Response`, and it will preserve
the overridden content type if you override it.

Fixes #14197, #14238

This partially reverts commit 30473768.

Setting this attribute to `true` will remove the content type header
from the request. This is use in `render :body` feature.

This is an option for sending a raw content back to browser. Note that
this rendering option will unset the default content type and does not
include "Content-Type" header back in the response.

You should only use this option if you are expecting the "Content-Type"
header to not be set. More information on "Content-Type" header can be
found on RFC 2616, section 7.2.1.

Please see #12374 for more detail.

Returning `self` from within the array returned by `to_ary`
caused this. Instead, we can just substitute another object.
It provides the `each` behavior required by the rack spec.

We can just require the file rather than going through the autoload
indirection

update Rails::Railtie::Configuration and ActionDispatch::Response#respond_to? to accept include_private argument

request thread to return up the stack before the instance variable is
assigned.  Synchronize so that the ivar is assigned before the other
thread can activate.

fixes #10984

After some discussion on Twitter with @skud, the documentation on ActionDispatch::Response is
a bit sparse. This class is useful when you're writing tests, as often you want to assert various
things about the response that's coming back. Better docs would make this easier for people new
to testing in Rails.

I only added some descriptions for various properties that were defined, and mostly just a sentence
or two. Most of these things are familliar if you're working with HTTP, but some words is better
than no words at all.

Hopefully further commits will fix up things that aren't just documentation.

[ci skip]

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      1) Failure:
    test_head_created_with_image_png_content_type(RenderTest) [test/controller/render_test.rb:1238]:
    Expected: "image/png"
      Actual: "image/png; charset=utf-8"