Specifically in rendering shortcuts, template responses, and class-based
views that return template responses.
Also added a test for render_to_response(status=...) which was missing
from fdbfc980.
Thanks Tim and Carl for the review.
This change preserves backwards-compatibility for a very common misuse
of render_to_response which even occurred in the official documentation.
It fixes that misuse wherever it happened in the code base and docs.
Context.__init__ is documented as accepting a dict and nothing else.
Since Context is dict-like, Context(Context({})) could work to some
extent. However, things get complicated with RequestContext and that
gets in the way of refactoring the template engine. This is the real
rationale for this change.
