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.

Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>