text: Title: Json decore error confluent platform
  Body: ## Checklist
  
  - [X] I use Confluent Kafka image
  - [X] I use a JSON schema for my topic
  ```json
  {
    "$schema": "http://json-schema.org/draft-04/schema#",
    "additionalProperties": false,
    "properties": {
      "frame": {
        "type": "string"
      },
      "time": {
        "format": "int64",
        "type": "integer"
      }
    },
    "required": [
      "time",
      "frame"
    ],
    "title": "RawData",
    "type": "object"
  }
  ```
  ## Steps to reproduce
  I have the following code :
  
  ```python
  import faust
  
  class RawData(faust.Record):
      frame: str
      time: int
  
  app = faust.App(
      id="test3",
      broker='kafka://localhost:9092',
      web_port=8080,
      processing_guarantee="exactly_once",
  )
  
  rawdata_topic = app.topic("RawDataSigfox")
  
  @app.agent(rawdata_topic)
  async def consumer(events):
      async for rd in events:
          print("rawdata", rd.time, rd.frame)
  
  
  if __name__ == '__main__':
      worker = faust.Worker(app, loglevel="INFO")
      worker.execute_from_commandline()
  ```
  I generate messages respecting the above schema from another service using confluent lib and schema registry and I wanted to read them from my python code.
  
  ## Expected behavior
  
  I expect to see something like `rawdata 1576320429 xxxxxxxx` in the console.
  
  ## Actual behavior
  
  It show me:
  faust.exceptions.ValueDecodeError: Expecting value: line 1 column 1 (char 0)
  
  If i don't use the `value_serializer="raw"`, it show me:
  
  Finding it weird, I add the property `value_serializer="raw"` to the topic.
  After that I get this:
  `b'\x00\x00\x00\x00\x16{"time":1576320429,"frame":"0000bb7465f504abff000000000000009902871600000000000000000b"}'`
  
  I tried to consume the topic directly with the confluent lib without going through faust, it manages to handle the message without worries.
  
  ## Full traceback
  [2020-08-28 19:35:53,980] [19848] [ERROR] [^----Agent*: __main__.consumer]: Crashed reason=ValueDecodeError('Expecting value: line 1 column 1 (char 0)',) 
  Traceback (most recent call last):
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\serializers\registry.py", line 99, in loads_value
      payload = self._loads(serializer, value)
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\serializers\registry.py", line 64, in _loads
      return loads(serializer, data)
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\serializers\codecs.py", line 368, in loads
      return get_codec(codec).loads(s) if codec else s
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\serializers\codecs.py", line 233, in loads
      s = cast(Codec, node)._loads(s)
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\serializers\codecs.py", line 258, in _loads
      return _json.loads(want_str(s))
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\utils\json.py", line 195, in loads
      return json_loads(s, **kwargs)
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\json\__init__.py", line 354, in loads
      return _default_decoder.decode(s)
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\json\decoder.py", line 339, in decode
      obj, end = self.raw_decode(s, idx=_w(s, 0).end())
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\json\decoder.py", line 357, in raw_decode
      raise JSONDecodeError("Expecting value", s, err.value) from None
  json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
  
  The above exception was the direct cause of the following exception:
  
  Traceback (most recent call last):
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\agents\agent.py", line 647, in _execute_actor
      await coro
    File "C:/Users/Bluebeel/Documents/Codes/Shayp/Shayp.Ingestion.Workers/app/app.py", line 18, in consumer
      async for rd in events:
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\streams.py", line 860, in _py_aiter
      raise chan_errors.popleft()
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\transport\conductor.py", line 95, in on_message
      event = await chan.decode(message, propagate=True)
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\serializers\schemas.py", line 146, in decode
      v: V = schema_loads_value(app, message, loads=loads_value)
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\serializers\schemas.py", line 85, in loads_value
      serializer=serializer or self.value_serializer,
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\serializers\registry.py", line 105, in loads_value
      sys.exc_info()[2]) from exc
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\serializers\registry.py", line 99, in loads_value
      payload = self._loads(serializer, value)
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\serializers\registry.py", line 64, in _loads
      return loads(serializer, data)
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\serializers\codecs.py", line 368, in loads
      return get_codec(codec).loads(s) if codec else s
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\serializers\codecs.py", line 233, in loads
      s = cast(Codec, node)._loads(s)
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\serializers\codecs.py", line 258, in _loads
      return _json.loads(want_str(s))
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\site-packages\faust\utils\json.py", line 195, in loads
      return json_loads(s, **kwargs)
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\json\__init__.py", line 354, in loads
      return _default_decoder.decode(s)
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\json\decoder.py", line 339, in decode
      obj, end = self.raw_decode(s, idx=_w(s, 0).end())
    File "C:\Users\Bluebeel\AppData\Local\Programs\Python\Python36\lib\json\decoder.py", line 357, in raw_decode
      raise JSONDecodeError("Expecting value", s, err.value) from None
  faust.exceptions.ValueDecodeError: Expecting value: line 1 column 1 (char 0)
  
  Process finished with exit code -1
  
  ```
  
  # Versions
  
  * Python 3.6.8
  * Faust 1.10.4
  * Windows 10
  * Kafka 2.5
  
  