我正在尝试在浏览器中进行三方授权，以便调用Twitter API。该过程始于通过向/oauth/request_token发出已签名的请求来获取请求令牌（这也是使用Twitter登录的方式之一）。
我的问题是，在浏览器POST到Twitter API端点之前，它想要使用OPTIONS方法preflight请求。但是，这个预检请求总是返回状态码400（错误请求）。
以下是一个示例，您可以将其复制并粘贴到支持Fetch API的浏览器控制台中：
fetch('https://api.twitter.com/oauth/request_token', { method: 'POST', mode: 'cors', headers: new Headers({ authorization: 'xxx' }), body: 'oauth_callback=http%3A%2F%2Flocalhost%2F' });
在Chrome上，预检请求看起来像这样（Firefox类似）：
OPTIONS /oauth/request_token HTTP/1.1
accept:*/*
accept-encoding:gzip, deflate, sdch
accept-language:en-US,en;q=0.8
access-control-request-headers:authorization, content-type
access-control-request-method:POST
cache-control:no-cache
origin:null
pragma:no-cache
user-agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/48.0.2564.116 Safari/537.36
预检响应看起来像这样：
HTTP/1.1 400 Bad Request
content-length: 0
date: Tue, 08 Mar 2016 22:21:37 GMT
server: tsa_a
x-connection-hash: 529e3d8338caeb980077637d86db5df1
请注意，问题不在于上面的示例中我没有指定真正的授权头信息。授权头信息的值不会在预检请求中使用。
如果我将我的POST请求的组成部分打印到控制台并将这些部分组装成一个curl命令（不进行预检），那么我可以获得一个请求令牌。但是，如果我尝试在curl中模拟预检请求，我就无法让它工作：
$ curl -v -X OPTIONS -H "access-control-request-headers:authorization,content-type" -H "access-control-request-method:POST" -H "origin:http://example.com" https://api.twitter.com/oauth/request_token
* Trying 199.59.148.20...
* Connected to api.twitter.com (199.59.148.20) port 443 (#0)
* ALPN, offering http/1.1
* Cipher selection: ALL:!EXPORT:!EXPORT40:!EXPORT56:!aNULL:!LOW:!RC4:@STRENGTH
* successfully set certificate verify locations:
* CAfile: /opt/local/share/curl/curl-ca-bundle.crt
CApath: none
* TLSv1.2 (OUT), TLS header, Certificate Status (22):
* TLSv1.2 (OUT), TLS handshake, Client hello (1):
* TLSv1.2 (IN), TLS handshake, Server hello (2):
* TLSv1.2 (IN), TLS handshake, Certificate (11):
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
* TLSv1.2 (IN), TLS handshake, Server finished (14):
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
* TLSv1.2 (OUT), TLS change cipher, Client hello (1):
* TLSv1.2 (OUT), TLS handshake, Finished (20):
* TLSv1.2 (IN), TLS change cipher, Client hello (1):
* TLSv1.2 (IN), TLS handshake, Finished (20):
* SSL connection using TLSv1.2 / ECDHE-ECDSA-AES128-GCM-SHA256
* ALPN, server accepted to use http/1.1
* Server certificate:
* subject: C=US; ST=CA; L=San Francisco; O=Twitter, Inc.; OU=Twitter Security; CN=api.twitter.com
* start date: Aug 11 00:00:00 2015 GMT
* expire date: Aug 15 12:00:00 2016 GMT
* subjectAltName: api.twitter.com matched
* issuer: C=US; O=DigiCert Inc; OU=www.digicert.com; CN=DigiCert SHA2 High Assurance Server CA
* SSL certificate verify ok.
> OPTIONS /oauth/request_token HTTP/1.1
> Host: api.twitter.com
> User-Agent: curl/7.47.1
> Accept: */*
> access-control-request-headers:authorization,content-type
> access-control-request-method:POST
> origin:http://example.com
>
< HTTP/1.1 400 Bad Request
< content-length: 0
< date: Tue, 08 Mar 2016 23:06:44 GMT
< server: tsa_a
< x-connection-hash: 66174829ef6d3f5e5ec641ac080ad19c
<
* Connection #0 to host api.twitter.com left intact
我缺少什么东西才能成功地进行CORS预检请求到https://api.twitter.com/oauth/request_token？