This reverts commit 4147ab73.

this commit removes some direct access to `env`.

This method is specifically about the content type so lets remove the
parameter.

create a singleton content type that just has nils, so that we don't
have to allocate a content type object all the time.

If someone sets just a charset, but depends on the implicit type from
rendering, this will store a strange content type header that looks like
this:  `; charset=blah`.  This is so that when the content type header
is parsed again, it will return nil for the actual type.

It turns out that the response object never really cares what the mime
type object is, so just use the string.

pull content-type setting to a private method to dry it up.

Instead of storing content type information in an ivar and a header,
lets move to just store the content type info in just the header.

we'll use this method later to lazily parse content type headers.

Just include the modules necessary in the Request object to implement
the things we need.  This should make it easier to build delegate
request objects because the API is smaller

eventually we'll remove this instance variable, but this is the first
step

we're storing the value in the headers hash, so lets just store the
value in one place.

Modules should be using the API that the abstract modules use so that we
can move these modules between implementations

cookies are always stored as a string in the header hash, so it will not
need to be joined.

eventually we will refactor this to not know about header hashes

Cleanup for `ActionDispatch::Http::Parameters` - no need for required libraries
and remove not used private method.

Apparently this method was used in `ActionDispatch::Http::Request` - fixed
by calling `Request::Utils` explicitly (as was done in other parts of the codebase)

Rack::Response::Helpers implements this method, so we can safely remove
it

Controllers should always have a request and response when responding.
Since we make this The Rule(tm), then controllers don't need to be
somewhere in limbo between "asking a response object for a rack
response" or "I, myself contain a rack response".  This duality leads to
conditionals spread through the codebase that we can delete:

  * https://github.com/rails/rails/blob/85a78d9358aa728298cd020cdc842b55c16f9549/actionpack/lib/action_controller/metal.rb#L221-L223

now the caller can just treat it like a regular controller even though
it will return a 404

We don't want to directly access the env hash

I want to implement this with something besides `@env` in the future, so
lets stop directly referencing it.

This commit allows us to use one request object rather than allocating
multiple request objects to deal with the session.

We're going to implement storing env values differently in the future,
so let's disconnect these methods from the instance variables

we're already doing this with a bunch of other header data.

we have a method that knows how to get rack.input, so lets use that.

We need to abstract the internals of the request object away from this
instance variable so that the values for `@env` can be calculated in a
different way.

This refactoring moves the controller class name that was on the route
set to the request. The purpose of this refactoring is for changes we
need to move controller tests to integration tests, mainly being able to
access the controller on the request instead of having to go through
the router.

[Eileen M. Uchitelle & Aaron Patterson]