Method constructor

tests/browser/headers.browser.test.js:6–18

()

class MockXMLHttpRequest {
  constructor() {
    this.requestHeaders = {};
    this.readyState = 0;
    this.status = 0;
    this.statusText = '';
    this.responseText = '';
    this.response = null;
    this.onreadystatechange = null;
    this.onloadend = null;
    this.upload = {
      addEventListener() {},
    };
  }

  open(method, url, async = true) {
    this.method = method;