This is another take at #14384 as we decided to wait until `master` is
targeting Rails 5.0. This commit is implementation-complete, as it
guarantees that all the public methods on the hash-inherited Parameters
are still working (based on test case). We can decide to follow-up later
if we want to remove some methods out from Parameters.

[ci skip] add note for individual stub creation

Rack [already implements `redirect?` on the response object](https://github.com/rack/rack/blob/1569a985e17d9caaf94d0e97d95ef642c4ab14ba/lib/rack/response.rb#L141) so we don't need to implement our own.

It is already on Active Support

Replace `ActiveSupport::Concurrency::Latch` with `Concurrent::CountDownLatch` from concurrent-ruby.

Change AC::TestResponse to AD::TestResponse

We want to treat the response object as if it's a real response object
(not a test object), so we should only call methods that are on the
superclass.

ActionController::TestResponse was removed in d9fe10cb and caused a test
failure on Action View as its test case still refers to it.

make test runner work correctly inside engine

We shouldn't depend on specific methods imlemented in the TestResponse
subclass because the response could actually be a real response object.

In the future, we should either push the aliased predicate methods in
TestResponse up to the real response object, or remove them

[ci skip] docs: making clear that perform_caching has a limited impact

This will save Travis some precious resource since it will not need to
run 18 jobs for these Ruby implementations on every push or pull
request.

We do care about these implementations but we should fix the build
locally before having it running on travis. We would love to have
someone working on this but right now it is not our reality.

The concurrent-ruby gem is a toolset containing many concurrency
utilities. Many of these utilities include runtime-specific
optimizations when possible. Rather than clutter the Rails codebase with
concurrency utilities separate from the core task, such tools can be
superseded by similar tools in the more specialized gem. This commit
replaces `ActiveSupport::Concurrency::Latch` with
`Concurrent::CountDownLatch`, which is functionally equivalent.

fix typo in caching guide [ci skip]

Removed usage line docs [ci skip]

Removes unnecessary comments from i18n validations tests [ci skip]

These comments do not add a lot to the readability, grepability or
overall understanding of the tests, therefore I believe they can be
safely removed.

fix class name typo [ci skip]

[ci skip] change sentence about reporting bugs in contributing guide

Improve duplicable documentation [ci skip]

Remove the reference to mocha in activemodel

Activemodel is no longer dependent on mocha, so we can make the comments
more generic.

constant loading should be thread safe now, so lets remove this