Fixed message labels in json format. (#42)

Signed-off-by: Pavel Kirilin <win10@list.ru>

Author: s3rius

taskiq/formatters/json_formatter.py

@@ -16,9 +16,7 @@ def dumps(self, message: TaskiqMessage) -> BrokerMessage:
             task_id=message.task_id,
             task_name=message.task_name,
             message=message.json(),
-            labels={
-                "content_type": "application/json",
-            },
+            labels=message.labels,
         )
 
     def loads(self, message: BrokerMessage) -> TaskiqMessage: