make sure form parameters are correctly encoded [REVIEW]

src/python3/ibisclient/connection.py

@@ -224,6 +224,7 @@ class IbisClientConnection:
         """
         path_params = self._params_to_strings(path_params)
         query_params = self._params_to_strings(query_params)
+        form_params = self._params_to_strings(form_params)
 
         url = "https://%s:%s%s" % (
             self.host, self.port,