Removed middleware setters.

I don't know if I can change other tests to make it work, but looks
the correct solution for me.

django/middleware/cache.py

@@ -70,14 +70,7 @@ def __init__(self, get_response):
 
     @property
     def cache(self):
-        try:
-            return self._cache
-        except AttributeError:
-            return caches[self.cache_alias]
-
-    @cache.setter
-    def cache(self, value):
-        self._cache = value
+        return caches[self.cache_alias]
 
     def _should_update_cache(self, request, response):
         return hasattr(request, '_cache_update_cache') and request._cache_update_cache
@@ -139,14 +132,7 @@ def __init__(self, get_response):
 
     @property
     def cache(self):
-        try:
-            return self._cache
-        except AttributeError:
-            return caches[self.cache_alias]
-
-    @cache.setter
-    def cache(self, value):
-        self._cache = value
+        return caches[self.cache_alias]
 
     def process_request(self, request):
         """
@@ -198,7 +184,6 @@ def __init__(self, get_response, cache_timeout=None, page_timeout=None, **kwargs
             self.key_prefix = key_prefix
         except KeyError:
             pass
-
         try:
             cache_alias = kwargs['cache_alias']
             if cache_alias is None:

tests/cache/tests.py

@@ -996,7 +996,6 @@ def test_custom_key_func(self):
 
     def test_cache_write_unpicklable_object(self):
         fetch_middleware = FetchFromCacheMiddleware(empty_response)
-        fetch_middleware.cache = cache
 
         request = self.factory.get('/cache/test')
         request._cache_update_cache = True
@@ -1011,7 +1010,6 @@ def get_response(req):
             return response
 
         update_middleware = UpdateCacheMiddleware(get_response)
-        update_middleware.cache = cache
         response = update_middleware(request)
 
         get_cache_data = fetch_middleware.process_request(request)