Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

On a Custom authentication scheme, try to access the request.stream as a stream.

Expected behavior

A stream instance is always returned

Actual behavior

A request instance is returned if request._read_started is false and a Stream is returned when it's true.

Example:

def authenticate(self, request):
    foo = request.body
    bar = request.stream
    value = request.stream.getvalue()

This works because request.stream returns a StringIO instance.

def authenticate(self, request):
    # foo = request.body
    # bar = request.stream
    value = request.stream.getvalue()

This doesn't because request.stream returns a Request instance (WSGIRequest in my case)

Removing this conditional solves the problem in this case, but i don't know what other implications it has.