Fix getFee returning value instead of failing when called twice

Author: federicobond

lib/transaction/transaction.js

@@ -899,13 +899,11 @@ Transaction.prototype._getOutputAmount = function() {
  */
 Transaction.prototype._getInputAmount = function() {
   if (_.isUndefined(this._inputAmount)) {
-    var self = this;
-    this._inputAmount = 0;
-    _.each(this.inputs, function(input) {
+    this._inputAmount = _.sumBy(this.inputs, function(input) {
       if (_.isUndefined(input.output)) {
         throw new errors.Transaction.Input.MissingPreviousOutput();
       }
-      self._inputAmount += input.output.satoshis;
+      return input.output.satoshis;
     });
   }
   return this._inputAmount;