Fix issue with batchorder endpoint (derivatvies)

- Fix an issue where orders are sent to the API endpoint
  (derivatives/api/v3/batchorder) but are not executed. It seems,
  according to kraken support, that the payload needs to be encoded
  (URL-encoding).

Author: pyantoine

src/kraken/futures/trade.py

@@ -10,6 +10,7 @@
 from __future__ import annotations
 
 from typing import TypeVar
+from urllib.parse import encode
 
 from kraken.base_api import FuturesClient, defined
 
@@ -237,7 +238,7 @@ def create_batch_order(
         return self.request(  # type: ignore[return-value]
             method="POST",
             uri="/derivatives/api/v3/batchorder",
-            post_params=params,
+            post_params=encode(params),
             auth=True,
             extra_params=extra_params,
         )