ajax – jqXHR.getAllResponseHeaders()不会返回所有标题

Connection: keep-alive
Content-Length: 64
Content-Type: application/json
X-My-CustomHeader: whatever

getAllResponseHeaders()仅返回：

Content-Type: application/json

我究竟做错了什么？

例

var request = {
  'url': 'http://api.someExternalDomain.com/resource/','type': someMethod,'success': function(data,textStatus,jqXHR) {
    console.log(jqXHR.getAllResponseHeaders());
  }
};

$.ajax(request);