xid in RootContext null xid in RpcContext null

- [ ] I have searched the [issues](https://github.com/seata/seata/issues) of this repository and believe that this is not a duplicate.

### Ⅰ. Issue Description
环境说明，本地订单服务、营销服务，nginx 做营销服务代理；
由订单服务调用营销服务，TX_XID能正常传递；
发布到CN环境，TX_XID无法传递，经过多方测试，是nginx环境对下划线请求头不能支持，
underscores_in_headers on;
设置局部开启下划线设置，本地测试OK；
再次CN 环境测试，依然无法得到分布式id。

### Ⅱ. Describe what happened

If there is an exception, please attach the exception trace:

```
Just paste your stack trace here!
临时解决方案，fegin拦截器中发送请求头过滤掉下划线（更换请求头参数key，再nginx中做取值还原seata原有设置）；
期望:分布式TX_XID可自定义或有降级传递措施，如：TX_XID -> TXXID -> XID

### Ⅵ. Environment:

- JDK version : 1.8
- OS : Linux
- Others: seata 1.2

Start with the Feign interceptor and the request-header path described in the issue, then reproduce the difference between the local setup and the CN environment with nginx. Compare propagation of TX_XID when underscores_in_headers is enabled and when it is not. Done should mean a distributed transaction ID remains transferable through the reported proxy setup, either with a supported custom header or a documented fallback.

Written by the indexing model from the issue text.