Beefed up the tests for multi-cache handling of the cache middleware and view...

Beefed up the tests for multi-cache handling of the cache middleware and view decorators, and made a couple of tweaks for edge cases as a result.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15021 bcc190cf-cafb-0310-a4f2-bffc1f526a37