convert FastbootResponse to es6 class

Author: bekzod

src/fastboot-response.js

@@ -1,8 +1,12 @@
+'use strict';
+
 var FastBootHeaders = require('./fastboot-headers');
 
-function FastbootResponse(response) {
-  this.headers = new FastBootHeaders(response._headers);
-  this.statusCode = 200;
+class FastbootResponse {
+  constructor(response) {
+    this.headers = new FastBootHeaders(response._headers);
+    this.statusCode = 200;
+  }
 }
 
 module.exports = FastbootResponse;