ActionDispatch::TestProcess::FixtureFile

This basically reverts e9fca766, d08da958,
d1fe1dcf, and 68eaf7b4

This reverts commit 3420a145, reversing
changes made to afb66a5a.

The rest of the helpers are better placed on Session -- and this is the
only one that cares which class it is defined on.

When passed an already-valid file name, prepending the path is likely to
create problems.

This is particularly relevant for #26384, which adds fixture_path
handling to test classes that previously didn't have it: any existing
caller must have been manually locating the file, and we don't want to
break them.

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

`with_indifferent_access` had been used in `assigns` method, but has been removed in ca83436d.

In Rails 5.1 `ActionController::TestCase` will be moved out of Rails
into it's own gem.

Please use `ActionDispatch::IntegrationTest` going foward.

Because this will be moved to a gem I used `# :stopdoc:` instead of
deleting the documentation. This will remove it from the Rails
documentation but still leave the method documented for when we move it
to a gem.

Guides have been updated to use the routing structure used in Integration
and all test examples have been updated to inherit from
`ActionDispatch::IntegrationTest` instead of `ActionController::TestCase.

Fixes #22496

eventually we will make the cookie jar derive these values from the
request object rather than save a reference to the values

Accessing a request object has nice advantages over accessing a hash.
If you use a missing method name, you'll get an exception rather than a
`nil` (is one nice feature)

It'd be a nice convention to mark the unused variables like this, now that Ruby 2 will issue no warnings for such vars being unused.

This allows test classes that are not subclasses of
ActiveSupport::TestCase (like those in rspec-rails) to interact with
with this variable without having to reference ActiveSupport::TestCase.

Assigning cookies for test cases should now use cookies[], e.g:

  cookies[:email] = 'user@example.com'
  get :index
  assert_equal 'user@example.com', cookies[:email]

To clear the cookies, use clear, e.g:

  cookies.clear
  get :index
  assert_nil cookies[:email]

We now no longer write out HTTP_COOKIE and the cookie jar is
persistent between requests so if you need to manipulate the environment
for your test you need to do it before the cookie jar is created.

- cookies can be set using string or symbol keys
- cookies are preserved across calls to get, post, etc.
- cookie names and values are escaped
- cookies can be cleared using @request.cookies.clear

[#6272 state:resolved]

Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

Signed-off-by: NXavier Noria <fxn@ubuntu.(none)>