`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]

[ci skip]

      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"

response in integration test accessible without @ via attr_reader

More info http://edgeguides.rubyonrails.org/api_app.html

[Carlos Antonio da Silva & Santiago Pastorino]

it was required on ruby 1.8

Fixes #3957

Fix previous commit by allowing a proc to be given as response_body. This is deprecated and is going to be removed in future releases.