Exposure of client IP addresses in net/http

Improper exposure of client IP addresses in net/http before Go 1.17.12 and Go 1.18.4 can be triggered by calling httputil.ReverseProxy.ServeHTTP with a Request.Header map containing a nil value for the X-Forwarded-For header, which causes ReverseProxy to set the client IP as the value of the X-Forwarded-For header.