Support passing None to test Client and RequestFactory request methods for default content_type

[Chris Jerdonek]

Currently, passing content_type=None to ​`test.Client.post()` results in the following error:

TypeError: expected string or bytes-like object

instead of causing the default content_type value. It would be more user-friendly if passing None resulted in the default value.

One reason is as follows. Currently, if one wants to create a wrapper function around post() with the same default behavior, the user either needs to (1) duplicate / hard-code the default value in their wrapper function, or (2) special-case None with a second invocation of post() using a different function signature (i.e. one that doesn't pass any content_type value). You can even see this happening in client.py with the way that Client.post() invokes RequestFactory.post() -- the default needs to be hard-coded twice.

[Tim Graham]

Should the same design be used for the other HTTP methods that have content_type='application/octet-stream'?

[Chris Jerdonek]

Oh, good observation. I didn't examine the other methods. Yes, I do.

Generally, I've found this type of style / pattern helpful anywhere an API has a default value. Perhaps other areas of Django could use a similar treatment. Might it be a candidate for the ​style guide?

[Chris Jerdonek]

I started working on this here: ​https://github.com/cjerdonek/django/tree/ticket_29268