The unit test introduced in 5bbcc128c for altering comment diff fragment

caching behavior on errors ended up being unstable. The Django function

we used to generate the cache header, patch_cache_control, generates a

dictionary of items and then turns it into a single string. This means

that the order of items is not in a stable sort order, causing our

checks to sometimes fail.
To fix this, we now split the Cache-Control header value, convert to a

set, and compare the items within. This avoids any sorting issues

going forward.

Unit tests pass.