Fix get_message_status #186 (#187)

Author: aliel

src/aleph/sdk/client/http.py

@@ -467,3 +467,5 @@ async def get_message_status(self, item_hash: str) -> MessageStatus:
             if resp.status == HTTPNotFound.status_code:
                 raise MessageNotFoundError(f"No such hash {item_hash}")
             resp.raise_for_status()
+            result = await resp.json()
+            return MessageStatus(result["status"])