Fixed #6371 - several decorators don't work with bound methods.

This involved changing the way the internal function
decorator_from_middleware works slightly, breaking some code that relied on
the old behaviour.  As a result, it is much simpler, but cache_page has been
made slightly more complex to cope with the change.



git-svn-id: http://code.djangoproject.com/svn/django/trunk@11586 bcc190cf-cafb-0310-a4f2-bffc1f526a37