Use the methods rack provides so we don't have to worry about the exact
header key.

Rack implements the Etag header manipulation methods, so we can use
those instead of ours.

Rails 4.x and earlier didn't support `Mime::Type[:FOO]`, so libraries
that support multiple Rails versions would've had to feature-detect
whether to use `Mime::Type[:FOO]` or `Mime::FOO`.

`Mime[:foo]` has been around for ages to look up registered MIME types
by symbol / extension, though, so libraries and plugins can safely
switch to that without breaking backward- or forward-compatibility.

Note: `Mime::ALL` isn't a real MIME type and isn't registered for lookup
by type or extension, so it's not available as `Mime[:all]`. We use it
internally as a wildcard for `respond_to` negotiation. If you use this
internal constant, continue to reference it with `Mime::ALL`.

Ref. efc6dd55

Just a slight refactor that delegates file sending to the response
object.  This gives us the advantage that if a webserver (in the future)
provides a response object that knows how to do accelerated file
serving, it can implement this method.

* Introduce `ActionDispatch::Http::Headers#add` to add a value to
  a multivalued header.
* Move `Response#add_header` upstream: https://github.com/rack/rack/pull/957
* Match upstream `Response#have_header?` -> `#has_header?` name change.

This reverts commit 4147ab73.

We only want to activate flash when the user has enabled it.  Api
servers don't use flash, so add an empty implementation to the base
Request object.

Committing the flash needs to happen in order for the session to be
written correctly, so lets guarantee that it actually does happen.

I'm doing this so that we can commit the flash to the session object Out
of Band of the flash middleware

Accessing mime types via constants is deprecated. Now, we are using `Mime::Type[:JSON]` instead of `Mime::JSON` 

Use the Rack utility methods for functional header manipulation.  This
helps to eliminate coupling on the header hash

I want to move the header hash to the super request object in order to
consolidate behavior.  We should be switching out buffering strategies
rather than header strategies since things like "mutating headers after
send" is an error in both cases (buffering vs streaming).

We should not mutate headers after the response has been committed.

I'm making this change so that I can construct response objects that
*don't* have the default headers applied.  For example, I would like to
construct a response object from the return value of a controller.

If you need to construct a response object with the default headers,
then please use the alternate constructor:
`ActionDispatch::Response.create`

As we all know that Accessing mime types via constants is deprecated. Now, we are using `Mime::Type[:JSON]` instead of `Mime::JSON`

The flash middleware shouldn't know how to look up the session object.
Just ask the request for that information.

We can know whether or not there is a content type object, and just exit
early.  There is no need to `try` so hard.

Synonyms are always a list of strings, and we have access to the
internal string representation, so we can avoid allocating new arrays.

Now that `all` has it's own object, we don't need the html_types Set.

This class gives us the `all?` predicate method that returns true
without hitting method missing

Since Mime::Type implements `method_missing`, and `blank?` triggers it's
positive branch:

  https://github.com/rails/rails/blob/f9dda1567ea8d5b27bd9d66ac5a8b43dc67a6b7e/actionpack/lib/action_dispatch/http/mime_type.rb#L342

We should stop calling `blank?`.

Changes `Mimes` to compose a set rather than inherit from array.  With
this change we don't need to define as many methods, so ISEQ memory is
saved.  Also it is clear which methods break the set cache.

We should be asking the mime type method for the mime objects rather
than via const lookup

We don't want to manage a list of constants on `Mime::`.  Managing
constants is strange because it will break method caches, not to mention
looking up by a constant could cause troubles.  For example suppose
there is a top level constant `HTML`, but nobody registers the HTML mime
type and someone accesses `Mime::HTML`.  Instead of getting an error
about how the mime type doesn't exist, instead you'll get the top level
constant.

So, instead of directly accessing the constants, change this:

  Mime::HTML

To this:

  Mime::Type[:HTML]

Now we don't have to look it up with a `const_get`.

forgotten end of the block

This is an instance method on the request object now so we don't need it
anymore

we don't actually need a param parser middleware instance since the
request object will take care of parsing parameters for us.  For now,
we'll just configure the parameter parsers on the request in this class.

The middleware stack is a singleton in the application (one instance is
shared for the entire application) which means that there was only one
opportunity to set the parameter parsers.  Since there is only one set
of parameter parsers in an app, lets just configure them on the request
class (since that is where they are used).

Parameters will not be parsed until they are specifically requested via
the `request_parameters` method.