Workaround breaking change in Django 3.0

Django 3.0 adds a default Content-Disposition header to all FileResponse
instances. Where the response isn't flagged as an attachment this header
has the form `inline; filename="<filename>"`. See:
https://code.djangoproject.com/ticket/30196

This header causes problems with Safari (see #240 and #241) so we want
to make sure we don't send it. Looking at the code for FileResponse I
can see that Django is also setting other default headers. These will
generally be overwritten by WhiteNoise, but they cause unnecessary file
accesses and provide more possibilities for errors.

This patch creates a tiny wrapper around FileResponse which disables the
setting of any default headers.

tests/test_django_whitenoise.py

@@ -12,12 +12,11 @@
 from django.core.management import call_command
 from django.utils.functional import empty
 
-from whitenoise.middleware import WhiteNoiseMiddleware
+from whitenoise.middleware import WhiteNoiseMiddleware, WhiteNoiseFileResponse
 
 from .utils import AppServer, Files
 
 
-
 @pytest.fixture(autouse=True, scope="module")
 def setup():
     yield django.setup()
@@ -142,6 +141,12 @@ def finder_static_files(request):
     return files
 
 
+def test_no_content_disposition_header(server, static_files, _collect_static):
+    url = settings.STATIC_URL + static_files.js_path
+    response = server.get(url)
+    assert response.headers.get("content-disposition") is None
+
+
 @pytest.fixture(scope="module")
 def finder_application(finder_static_files):
     return get_wsgi_application()
@@ -195,3 +200,12 @@ def test_directory_path_without_trailing_slash_redirected(
     location = get_url_path(response.url, response.headers["Location"])
     assert response.status_code == 302
     assert location == settings.STATIC_URL + directory_path
+
+
+def test_whitenoise_file_response_has_only_one_header():
+    response = WhiteNoiseFileResponse(open(__file__, "rb"))
+    response.close()
+    headers = {key.lower() for key, value in response.items()}
+    # This subclass should have none of the default headers that FileReponse
+    # sets
+    assert headers == {"content-type"}

whitenoise/middleware.py

@@ -14,6 +14,18 @@
 __all__ = ["WhiteNoiseMiddleware"]
 
 
+class WhiteNoiseFileResponse(FileResponse):
+    """
+    Wrap Django's FileResponse to prevent setting any default headers. For the
+    most part these just duplicate work already done by WhiteNoise but in some
+    cases (e.g. the content-disposition header introduced in Django 3.0) they
+    are actively harmful.
+    """
+
+    def set_headers(self, *args, **kwargs):
+        pass
+
+
 class WhiteNoiseMiddleware(WhiteNoise):
     """
     Wrap WhiteNoise to allow it to function as Django middleware, rather
@@ -58,7 +70,7 @@ def process_request(self, request):
     def serve(static_file, request):
         response = static_file.get_response(request.method, request.META)
         status = int(response.status)
-        http_response = FileResponse(response.file or (), status=status)
+        http_response = WhiteNoiseFileResponse(response.file or (), status=status)
         # Remove default content-type
         del http_response["content-type"]
         for key, value in response.headers: