Ensure cookies are passed.
Move `rack.session` into request headers.
TEMPFIX: We have to replace the `app` routes now that we are inheriting from
integration tests.
Remove support for AnonymousController.
Remove a bunch of old code.
Avoid manipulating parameters.

Pass them straight to integration tests.
Revert "Don't paramify ActionDispatch::Http::UploadedFile in tests"

This reverts commit 8471edc6725aa905dc9f7b800438acb255b1512e.

Ensure proper testing through `fixture_file_upload`.
fixup! WIP fixed to test_case_test.
Update test to be consistent with Rack.
Allow `QUERY_STRING` to be manipulated.
Clear `QUERY_STRING` after each request.
Ensure that Response#content_type is converted to string.
Fix `action_controller.instance` not being able to convert to JSON.