why "connection: close" header is added when the request is passed to upstream server?

[tjlp at sina.com]

 Hi, Nginx guy,

I use Nginx in the Kubernetes. With the upstream server log, I find that the header "connection: close" is 
added when the request is passed to upstream server. Why? What I hope is the original header relating to 
connection status should be passed to upstream server without any 
change. That means: if the original request header has no connection header, "connection" header should not be added. 

The final expected behavior is: if no connection header in the request header, the session should be keep-alive, if there is "connection: close" header, the session should be terminated after the response is returned.
Thanks
Liu Peng
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