Fix request body/POST access

[rpkilby]

Supersedes/closes #5583 and fixes #5582.

[rpkilby]

cc @D3X, @tomchristie.

Hi Tom, see my comment here for context. It seems like setting request._post in addition to request._files might have been the correct thing to do. Thoughts?

[D3X]

@rpkilby One thing I'm concerned about is that this changes the contract on request._post.

Normally, Django would set it to an empty QueryDict if the Content-Type of the request was not "application/x-www-form-urlencoded" or "multipart/form-data".

With this PR, it gets set to parsed data even for other types, like "application/json". This could result in not-easy-to-track bugs if some middlewares depended on Django's behaviour in this case.

[xordoquy]

This shouldn't impact middlewares as they are called with Django's request. DRF's views does the wrapping so it's really used "inside" DRF whether core or the application code.

[D3X]

@xordoquy I wrote a short test to show how the interface on request.POST changes when using DRF and posting JSON: D3X@aa6a4c4

I don't know any app that would break because of it, but it is possible some would.

[rpkilby]

@D3X - thanks, good catch.

[carltongibson]

@rpkilby This looks fine to me — I'm inclined to just pull it in, but I'm not quite seeing how the commits match the conversation. (Did you rebase? What changed?)

I wrote a short test to show how the interface on request.POST changes...

OK, added, but what was this line in the first version?

     self._request._post = self.POST

Thanks!

[rpkilby]

@carltongibson - yeah, I amended the commits from earlier in the conversation. Previously was

     self._request._post = self._data
     self._request._files = self._files

[carltongibson]

@rpkilby OK, thanks. That makes perfect sense.

[carltongibson]

OK, let’s have this. Thanks @D3X @rpkilby @xordoquy!

[xordoquy]

I have no idea how this PR works.
_load_data_and_files call POST which may call _load_data_and_files.

[rpkilby]

@xordoquy - it's definitely weird, but it won't reenter. eg, if you call request.POST, it will trigger the call to _load_data_and_files(). At this point, _data/_files/_full_data are set, so the inner call to request.POST won't call _load_data_and_files() again.