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.

* first test is for `default_charset` i.e `ActionDispatch::Response.default_charset = “utf-8”`

* In below test we are passing `ActionDispatch::Response.default_charset = 'utf-16’` so name of the test is irrelevant — “read content type without charset”

Now, we use the mapper to build the routing table

related to https://github.com/rails/rails/commit/703275ba70efbefb3358052b6ba750443eff1a28

the caller of `handle_conditional_get!` checks the committed state of
the response, so we don't need to in the subclass.

This reverts commit 4147ab73.

* add `end` to end of class definition
* add a blank line between explanation and example code

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.

we want the request to go through the test harness, not directly call
the methods on the controller

also remove req / res references

again, since we are going through the test harness, all this is done
for us.

Since we just go through the normal test harness that sets up a request
for us, we don't need to do this anymore.

rather than calling methods on the controller.  We should test the
values returned by the controller rather than assuming that the
internals are implemented in a certain way.

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`

It doesn't make sense to access the response object before a request is
made (how was a response object created without making a request?)  This
commit splits testing default headers and default header mutation tests
and removes access to the pre-request response object.

these should really be multiple tests.

When the response object is `to_a`'d, that means it's been written to
the socket.  It doesn't make sense to mutate the response object after
it's been written (and this may raise an exception in the future).