Make backends store bytes instead of strings

This is, for the majority of backends, the native format anyway, and so
we save encoding and decoding when using the PickleCodec or if (in future)
a orjson Coder was to be added.

For the JsonCodec, the only thing that changed is the location where the
JSON data is encoded to bytes and decoded back again to a string.

README.md

@@ -78,7 +78,7 @@ async def index():
 
 @app.on_event("startup")
 async def startup():
-    redis = aioredis.from_url("redis://localhost", encoding="utf8", decode_responses=True)
+    redis = aioredis.from_url("redis://localhost")
     FastAPICache.init(RedisBackend(redis), prefix="fastapi-cache")
 
 ```
@@ -180,6 +180,13 @@ async def index():
 `InMemoryBackend` store cache data in memory and use lazy delete, which mean if you don't access it after cached, it
 will not delete automatically.
 
+
+### RedisBackend
+
+When using the redis backend, please make sure you pass in a redis client that does [_not_ decode responses][redis-decode] (`decode_responses` **must** be `False`, which is the default). Cached data is stored as `bytes` (binary), decoding these i the redis client would break caching.
+
+[redis-decode]: https://redis-py.readthedocs.io/en/latest/examples/connection_examples.html#by-default-Redis-return-binary-responses,-to-decode-them-use-decode_responses=True
+
 ## Tests and coverage
 
 ```shell