collect the body and make assert against the collected body

We should keep the route set generation logic in one place

The tests and methods were hard to read with `options[:options]` all
over the place. This refactoring makes the code easier to understand.

The change came out of work for moving the underlying code of controller
tests to integraiton tests.

Looks like this was left over from converting Rails to Rack.  I think
it's safe to remove now.

since we only work with instances of classes, it greatly simplifies the
`Middleware` implementation.

 - webservers do not do it
 - it makes redirect urls ugly when request.host is used for redirection

This clears the transaction record state when the transaction finishes
with a `:committed` status.

Considering the following example where `name` is a required attribute.
Before we had `new_record?` returning `true` for a persisted record:

```ruby
  author = Author.create! name: 'foo'
  author.name = nil
  author.save        # => false
  author.new_record? # => true
```

When executing an `ActionController::Parameters#fetch` with a block
that raises a `KeyError` the raised `KeyError` will be rescued and
converted to an `ActionController::ParameterMissing` exception,
covering up the original exception.

[Jonas Schubert Erlandsson & Roque Pinel]

This will silence deprecation warnings.

Most of the test can be changed from `render :text` to render `:plain`
or `render :body` right away. However, there are some tests that needed
to be fixed by hand as they actually assert the default Content-Type
returned from `render :body`.

We've started on discouraging the usage of `render :text` in #12374.
This is a follow-up commit to make sure that we print out the
deprecation warning.

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.

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.

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.

We should use rack-test's upload file objects on the test side so that
we will be able to correctly generate mime blob posts in the future

We should convert request parameters to a query string, then let the
request object parse that query string.  This should give us results
that are more similar to the real-world

We should assign parameters to the request object rather than mutate the
hash that is returned by `query_parameters` or `request_parameters`

Instead of trying to manually clear out a request object, lets just
allocate a new one.  The rack ENV is reused and cleaned (still), but the
request object is not.

We should leverage the request / response objects that the superclass
has already allocated for us.

Variants are typically set in the controller based on some attribute of
the request that the browser sent.  We should make our tests more in
line with reality by doing the same and not mutating the request object.

In 0de4a23d the behavior when there is a missing template was changed to
not raise an error, but instead head :no_content.  This is a breaking
change and some gems rely on this happening.

To allow gems and other code to work around this, allow
`default_render` to take a block which, if provided, will
execute the contents of that block instead of doing the `head :no_content`.