Add support to enhance the cacheability of course assets.

This introduces a mechanism to control the time-to-live for an unlocked
course asset, which will allow browsers and intermediate proxies/caches
to cache these course assets, determinstically.

Locked assets, with their nature of requiring authorization, are not
eligible for caching.

common/djangoapps/clean_headers/tests/test_decorators.py

@@ -0,0 +1,20 @@
+"""Tests for clean_headers decorator. """
+from django.http import HttpResponse, HttpRequest
+from django.test import TestCase
+from clean_headers.decorators import clean_headers
+
+
+def fake_view(_request):
+    """Fake view that returns an empty response."""
+    return HttpResponse()
+
+
+class TestCleanHeaders(TestCase):
+    """Test the `clean_headers` decorator."""
+
+    def test_clean_headers(self):
+        request = HttpRequest()
+        wrapper = clean_headers('Vary', 'Accept-Encoding')
+        wrapped_view = wrapper(fake_view)
+        response = wrapped_view(request)
+        self.assertEqual(len(response.clean_headers), 2)

common/djangoapps/clean_headers/tests/test_middleware.py

@@ -0,0 +1,34 @@
+"""Tests for clean_headers middleware."""
+from django.http import HttpResponse, HttpRequest
+from django.test import TestCase
+from clean_headers.middleware import CleanHeadersMiddleware
+
+
+class TestCleanHeadersMiddlewareProcessResponse(TestCase):
+    """Test the `clean_headers` middleware. """
+    def setUp(self):
+        super(TestCleanHeadersMiddlewareProcessResponse, self).setUp()
+        self.middleware = CleanHeadersMiddleware()
+
+    def test_cleans_intended_headers(self):
+        fake_request = HttpRequest()
+
+        fake_response = HttpResponse()
+        fake_response['Vary'] = 'Cookie'
+        fake_response['Accept-Encoding'] = 'gzip'
+        fake_response.clean_headers = ['Vary']
+
+        result = self.middleware.process_response(fake_request, fake_response)
+        self.assertNotIn('Vary', result)
+        self.assertEquals('gzip', result['Accept-Encoding'])
+
+    def test_does_not_mangle_undecorated_response(self):
+        fake_request = HttpRequest()
+
+        fake_response = HttpResponse()
+        fake_response['Vary'] = 'Cookie'
+        fake_response['Accept-Encoding'] = 'gzip'
+
+        result = self.middleware.process_response(fake_request, fake_response)
+        self.assertEquals('Cookie', result['Vary'])
+        self.assertEquals('gzip', result['Accept-Encoding'])