validate that routes are an array before merging with another Router instance

src/router.ts

@@ -95,6 +95,8 @@ export default class Router {
    */
   use(obj: Router | Tpl): void {
     if(obj instanceof Router) {
+      if(!Array.isArray(obj.routes))
+        throw new TypeError("Routes must be an array.");
       this.routes = [ ...this.routes, ...obj.routes ];
       this.sortRoutes();
     }