这似乎不起作用：

$.ajax({
    url:      "http://localhost:3000/foo.json",
    data:     { foo: 'bar' },
    headers:  { 'HTTP_X_CUSTOMHEADER': 'foobar' },
    xhrFields: { withCredentials: true }
});

当我在jsfiddle上运行它时，将触发一个OPTIONS请求（根据Chrome调试工具），如下所示：

Access-Control-Request-Headers: Origin, HTTP_X_CUSTOMHEADER, Accept
Access-Control-Request-Method:  GET
Origin:                         http://fiddle.jshell.net

然后（根据Chrome调试工具）我的本地服务器返回以下标头：

（手动重新格式化以提高可读性）

Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers:     HTTP_X_CUSTOMHEADER
Access-Control-Allow-Methods:     GET, POST, PUT, DELETE, OPTIONS
Access-Control-Allow-Origin:      http://fiddle.jshell.net
Access-Control-Max-Age:           10

Cache-Control:                    no-cache
Connection:                       Keep-Alive
Content-Length:                   1
Content-Type:                     text/html; charset=utf-8
Date:                             Wed, 14 Sep 2011 22:42:28 GMT
Server:                           WEBrick/1.3.1 (Ruby/1.8.7/2010-01-10)
X-Runtime:                        2

然后在控制台中，我收到如下错误消息：

XMLHttpRequest cannot load http://localhost:3000/foo.json?foo=bar.
Origin http://fiddle.jshell.net is not allowed by Access-Control-Allow-Origin.

但是Access-Control-Allow-Origin标题与我的服务器响应预检请求时显示的标题相同。那么，我在这个难题中遗漏了什么？