Ensure cloudfront purges are called with lists

wagtail/contrib/frontend_cache/backends/cloudfront.py

@@ -72,7 +72,7 @@ class CloudfrontBackend(BaseBackend):
                 paths_by_distribution_id[distribution_id].add(url_parsed.path)
 
         for distribution_id, paths in paths_by_distribution_id.items():
-            self._create_invalidation(distribution_id, paths)
+            self._create_invalidation(distribution_id, list(paths))
 
     def purge(self, url):
         self.purge_batch([url])

wagtail/contrib/frontend_cache/tests.py

@@ -360,7 +360,7 @@ class TestBackendConfiguration(SimpleTestCase):
             backends.get("cloudfront").purge("http://torchbox.com/blog/")
 
         _create_invalidation.assert_called_once_with(
-            "frontend", {"/home/events/christmas/"}
+            "frontend", ["/home/events/christmas/"]
         )
 
         self.assertTrue(