Decorator creates cache with Cache() constructor

* See aio-libs#473: Deprecate using cache specific constructors
  * aio-libs#473
  * `Cache` class was introduced as a proxy for instantiating the different
    backends. There should be only one way of doing things and `Cache` is
    the preferred way of doing that so will deprecate the others.
* This is now applied to `cached._get_cache()`

aiocache/decorators.py

@@ -205,7 +205,12 @@ async def decorator(self, f, *args, **kwargs):
 
 
 def _get_cache(cache=Cache.MEMORY, serializer=None, plugins=None, **cache_kwargs):
-    return cache(serializer=serializer, plugins=plugins, **cache_kwargs)
+    return Cache(
+        cache, 
+        serializer=serializer, 
+        plugins=plugins, 
+        **cache_kwargs,
+    )
 
 
 def _get_args_dict(func, args, kwargs):