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This is not implemented using a `def head()` in the view, as the HEAD request
should return all headers that the GET request would send. Hence,
updated the server logic to provide a proper HEAD request.

Fix spelling error

 Support X-Accel-Redirect quoting in nginx >=1.5.9

This assumes most people are on nginx versions newer than 1.5.9

Newer versions of nginx require a properly encoded URI in the X-Accel-Redirected response. According to research done by people on the django-sendfile issue tracker, the specific version number seems to be 1.5.9.

The ``PRIVATE_STORAGE_NGINX_VERSION`` setting is used to determine the proper encoding (defaults to current behaviour of not quoting).

https://github.com/adnrs96/django-sendfile/commit/4488e520f04661136e88be38281e789ae50a260c

Renamed test paths for uniqueness.
Show which files do exist in the folder.

- Support upload_to parameter (#3, #13)
- Support upload_subfolder returning strings instead of lists (#5, #11)
- Avoid restoring the deprecated get_directory_name() and get_filename()
  functions that were deprecated as of Django 1.0 and removed in Django 2.0
  Instead, generate_filename() was updated to support the recent Django
  behavior

Django 2.0 experimental support

Using user.is_authenticated() and user.is_anonymous() as a method is deprecated since Django 2.0