Fixed #11857 -- Added missing 'closed' property on TemporaryFile class.

- TemporaryFile now minimally mocks the API of the Python standard
  library class tempfile.NamedTemporaryFile to avoid AttributeError
  exceptions.
- The symbol django.core.files.NamedTemporaryFile is actually assigned
  as a different class on different operating systems.
- The bug only occurred if Django is running on Windows, hence why it
  was hard to diagnose.