`HistoryRequestMiddleware` doesn't clean up after itself under some circumstances

[manelclos]

Describe the bug
We caught a problem were the request was reuse, causing a strange random behaviour.

To Reproduce
Steps to reproduce the behavior:

• a test makes a request, so the context is updated
• an exception happens and process_response is never called, context still contains WRONG request
• the next test creates some versioned model, and the WRONG request is used

Expected behavior
Simple history not using the previous request.

Environment (please complete the following information):

• OS: [docker python:3.8 image]
• Django Simple History Version: 3.3.0
• Django Version: 2.2.
• Database Version: MySQL 8.0

Pull request here: #1188

Will complete the checklist if you think this should go in.