Some minor changes to the `patch_vary_headers` function:

 * Replaced a for loop with a list comprehension.
 * Used a set instead of a dict with dummy values.
 * Used a bit more readable variable names.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@6698 bcc190cf-cafb-0310-a4f2-bffc1f526a37