code: function MockRes (url, cb) {
      this.url = url;
      this.headers = {};
      this.content = undefined;
      this.setHeader = function (name, value) {
          this.headers[name] = value;
      }.bind(this);
      this.end = function (content) {
          this.content = content;
          cb(this);
      }.bind(this);
  }
docs: Mocked response
  @param {String} url
  @param {Function} cb Callback which is passed file content (Buffer)
  @constructor