- Deprecated all assignments to ``request.response_*`` attributes such as
  ``request.response_content_type = 'foo'``.  Assignments and mutations of
  the following request attributes that were considered by the framework for
  response influence are now deprecated: ``response_content_type``,
  ``response_headerlist``, ``response_status``, ``response_charset``, and
  ``response_cache_for``.  Instead of assigning these to the request object
  for detection by the rendering machinery, users should use the appropriate
  API of the Response object created by accessing ``request.response``
  (e.g. ``request.response_content_type = 'abc'`` ->
  ``request.response.content_type = 'abc'``).

- Custom request objects are now required to have a ``response`` attribute
  (or reified property) if they are meant to be used with renderers.  This
  ``response`` attribute should be an instance of the class
  ``pyramid.response.Response``.

- The JSON and string renderer factories now use
  ``request.response.content_type`` rather than
  ``request.response_content_type``.  They determine whether they should set
  the content type of the response by comparing the response's content type
  against the default (usually ``text/html``); if the content type is not the
  default, the renderer changes the content type (to ``application/json`` or
  ``text/plain`` for JSON and string renderers respectively).

- Made it possible to assign to and delete
  ``pyramid.testing.DummyRequest.registry`` (bugfix).