fix(oauth): fix private key error for OAuth tokens

ts/bunq.classes.session.ts

@@ -149,6 +149,8 @@ export class BunqSession {
       // OAuth tokens don't expire in the same way as regular sessions
       // Set a far future expiry time
       this.sessionExpiryTime = plugins.smarttime.TimeStamp.fromMilliSeconds(Date.now() + 365 * 24 * 60 * 60 * 1000);
+      // Also set OAuth mode on HTTP client
+      this.httpClient.setOAuthMode(true);
     }
   }