Bitfinex don't submit Stop order at margin account

[MatheusKlauck]

Bitfinex API don't submit stop orders to the margin account.
When you try to send a stop order to a margin account the code automatically choose switch to "margin" or "market" order type.

[vslee]

Thanks for filing this bug. If you send a pull request with the solution, we'll be happy to merge it. If you don't know how, you can also post the updated code here.