Fix LoggedModel.thread.request not being cleared in failing tests

[richardebeling]

The deletion was not working for failing tests. This caused the setup logic for following tests to assume that the "old" request.user is responsible for the current change (usually a baker.make() call), while the user object didn't even exist in the database anymore. In consequence, it caused foreign key integrity errors for tests that are totally unrelated to the actual failure.

This is interesting because it contradicts (my interpretation) of the django documentation, which reads as if just running teardown code after calling self.get_response(request) should be fine, because there should be some wrapper that catches exceptions and turns them into 400 or 500 responses. Maybe this does not apply when running tests with our setup so we can get meaningful stack traces?

Anyway, it doesn't look to me as if this would be critical for any other middleware. The language middleware might not correctly set the language cookie, but that shouldn't be an issue.

[niklasmohrin]

In the exception handling for responses, I found that there is a setting DEBUG_PROPAGATE_EXCEPTIONS which is read in handle_uncaught_exception in core.handlers.exception and re-raises. Webtest activates this setting WebtestMixin._patch_settings