[FIX] get interest for currency without interest

- `get_interest_at` for a currency without interest would not hit the intended check since the function was used as variable instead of calling the method

src/js/ripple/currency.js

@@ -270,7 +270,7 @@ Currency.prototype.has_interest = function() {
  * @returns {number} - interest for provided interval, can be negative for demurred currencies
  */
 Currency.prototype.get_interest_at = function(referenceDate, decimals) {
-  if (!this.has_interest) {
+  if (!this.has_interest()) {
     return 0;
   }