CacheMiddleware should respect max-age

For example (assuming you have the middleware turned on):

from django.views.decorators.cache import never_cache, cache_control

def test_cache(request):
    from datetime import datetime
    from django.http import HttpResponse
    return HttpResponse(str(datetime.now()))

cached_view = test_cache
no_cache_view = never_cache(test_cache)
short_cache_view = cache_control(max_age=5)(test_cache)

When/if this is committed it should probably be noted on BackwardsIncompatibleChanges that the per-site cache logic has changed.

oops, that was me.

(In [6736]) Fixed #5813 -- Taught the CacheMiddleware? to respect any max-age HTTP header when setting the expiry time. Thanks, SmileyChris?.