can be called with `returnOpenOrders('all', callback)`
function (currencyA, currencyB, callback) {
      if (typeof currencyB === 'function') {
        callback = currencyB;
        currencyB = null;
      }

      const parameters = {
        currencyPair: joinCurrencies(currencyA, currencyB)
      };

      return this._private('returnOpenOrders', parameters, callback);
    }