Clear ThreadLocal request atttribute after response is processed

[lucaswiman]

@jedie This pull request updates the ThreadLocal middleware so that the thread local reference to the request object is cleared when the response is processed by the middleware.

Issue Description

This is unlikely to matter in practice since a server will overwrite the previous _thread_locals.request when it receives a new request.

The use case this is intended to fix is:

• A function f() is used on both the front end and back end of an application.
• It collects metadata when used on the front end, for example to record an audit log of user actions. If get_current_{request,user} is called, it creates a model entry with a foreign key referencing the change the user made.
• When used on the backend, no model entry M is created, or the User foreign key is NULL.
• A test suite runs:
  • It first tests the front-end audit logging, using a simulated request from django's test client, which sets the _thread_locals.request variable. The User object is created as part of the test, then destroyed when the database transaction is rolled back by the tearDown method.
  • It subsequently runs a test of the back end functionality, which attempts to save an audit log of the action from the user on the previous test. Since this user no longer exists, a database integrity error is raised.

Changes

• Adds a ThreadLocalMiddleware.process_response method, which is part of the Django middleware API: https://docs.djangoproject.com/en/1.8/topics/http/middleware/#process-response This method clears the request variable if it is defined.
• Corrects some typos in the ThreadLocal module's documentation.

Testing

I added two views to the example app. One that returns the GET parameters from the current request, and one that simply raises an exception. I added tests that assert:

• The current GET parameters are actually returned (i.e. that get_current_request actually returns the current request). This passed on the master branch, and is just included for completeness.
• When a successful request is executed, the request reference is cleared. This failed using the code on the master branch.
• When an unsuccessful request is executed (in the raise_exception view), the request reference is cleared. This failed using the code on the master branch.

Thanks for maintaining this library!

[jedie]

Looks great!

Please add you to AUTHORS...

[lucaswiman]

Added to AUTHORS in c60c9c8. Should I update the changelog as well?

[landscape-bot]

Repository health decreased by 0.13% when pulling c60c9c8 on lucaswiman:master into d3f784f on jedie:master.

• 3 new problems were found (including 0 errors and 3 code smells).
• No problems were fixed.

[jedie]

Should I update the changelog as well?

I will do this