futures: fix cancel_all_open_orders() reqwest decode () error

[eliotOrderson]

cancel_all_open_orders can successful cancel all open orders, But it will happened the error: " ReqError(reqwest::Error { kind: Decode, source: Error("invalid type: map, expected unit", line: 1, column: 0) }) ", I don't know why this is happening, But after specifying T for "Success", the error will be resolved