function Response(req, options) {
  options = options || {};
  this.req = req;
  this.xhr = this.req.xhr;
  // responseText is accessible only if responseType is '' or 'text' and on older browsers
  this.text = ((this.req.method !='HEAD' && (this.xhr.responseType === '' || this.xhr.responseType === 'text')) || typeof this.xhr.responseType === 'undefined')
     ? this.xhr.responseText
     : null;
  this.statusText = this.req.xhr.statusText;
  this._setStatusProperties(this.xhr.status);
  this.header = this.headers = parseHeader(this.xhr.getAllResponseHeaders());
  // getAllResponseHeaders sometimes falsely returns "" for CORS requests, but
  // getResponseHeader still works. so we get content-type even if getting
  // other headers fails.
  this.header['content-type'] = this.xhr.getResponseHeader('content-type');
  this._setHeaderProperties(this.header);
  this.body = this.req.method != 'HEAD'
    ? this._parseBody(this.text ? this.text : this.xhr.response)
    : null;
}
Initialize a new `Request` with the given `method` and `url`.

@param {String} method
@param {String} url
@api public