ADD: preimage in /gettxs response

class/User.js

@@ -311,9 +311,11 @@ export class User {
         invoice.timestamp = invoice.decoded.timestamp;
         invoice.memo = invoice.decoded.description;
       }
+      if (invoice.payment_preimage) {
+        invoice.payment_preimage = Buffer.from(invoice.payment_preimage, 'hex').toString('hex');
+      }
       // removing unsued by client fields to reduce size
       delete invoice.payment_error;
-      delete invoice.payment_preimage;
       delete invoice.payment_route;
       delete invoice.pay_req;
       delete invoice.decoded;