This reverts commit 90c887fa.

Selecting which key extensions to include in active_support/rails
made apparent the systematic usage of Object#in? in the code base.
After some discussion in

    https://github.com/rails/rails/commit/5ea6b0df9a36d033f21b52049426257a4637028d

we decided to remove it and use plain Ruby, which seems enough
for this particular idiom.

In this commit the refactor has been made case by case. Sometimes
include? is the natural alternative, others a simple || is the
way you actually spell the condition in your head, others a case
statement seems more appropriate. I have chosen the one I liked
the most in each case.

- #respond_to's documentation refer to .respond_to, but it was
  written as just <respond_to>. Added class name for clarification.
 

This reverts commit fa6d921e.

Reason: Not a fan of such massive changes. We usually close such changes
if made to Rails master as a pull request. Following the same principle
here and reverting.

[ci skip]

For future reference, this is the regex I used: ^\s*#\s*\n(?!\s*#). Replace
with the first match, and voilà! Note that the regex matches a little bit too
much, so you probably want to `git add -i .` and go through every single diff
to check if it actually should be changed.

Raise a rescuable exception when Rails doesn't know what to do with the format, rather than responding with a head :not_acceptable (406)

We should let the responder to decide what to do with the given
overridden response block, and not short circuit it.

Fixes #5280

Closes #5025 part 2

Default responder was only using the given respond block when user
requested for HTML format, or JSON/XML format with valid resource. This
fix the responder so that it will use the given block regardless of the
validity of the resource. Note that in this case you'll have to check
for object's validity by yourself in the controller.

Fixes #4796

There're a lot of places in Rails source code which make a lot of sense to switching to Object#in? or Object#either? instead of using [].include?.

So we don't miss out on the nice documentation of the respond_to and
respond_with instance methods.  Also added :nodoc: to protected method.

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Explain in the method doc that you need to call respond_to at the class level in order to use respond_with.

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