caddyhttp: Restore original request params before error handlers (#3781)

* caddyhttp: Restore original request params before error handlers

Fixes #3717

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Author: mholt

modules/caddyhttp/server.go

@@ -209,6 +209,14 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// restore original request before invoking error handler chain (issue #3717)
+	// TODO: this does not restore original headers, if modified (for efficiency)
+	origReq := r.Context().Value(OriginalRequestCtxKey).(http.Request)
+	r.Method = origReq.Method
+	r.RemoteAddr = origReq.RemoteAddr
+	r.RequestURI = origReq.RequestURI
+	cloneURL(origReq.URL, r.URL)
+
 	// prepare the error log
 	logger := errLog
 	if s.Logs != nil {