Allow redirect middleware HTTP responses to be overridden  (ticket #19321)

This pull request adds tests for Aymeric's redirects middleware patch added in [ticket 19321](https://code.djangoproject.com/ticket/19321).

Do we want to document this? There was some hesitation expressed in [ticket 19277](https://code.djangoproject.com/ticket/19277) and ultimately no docs were changed.