proxy: recognize client's cancellation

fix issue #1589

Signed-off-by: Tw <tw19881113@gmail.com>

caddyhttp/proxy/reverseproxy.go

@@ -12,7 +12,6 @@
 package proxy
 
 import (
-	"context"
 	"crypto/tls"
 	"io"
 	"net"
@@ -252,14 +251,6 @@ func (rp *ReverseProxy) ServeHTTP(rw http.ResponseWriter, outreq *http.Request,
 
 	rp.Director(outreq)
 
-	// Original incoming server request may be canceled by the
-	// user or by std lib(e.g. too many idle connections).
-	// Now we issue the new outgoing client request which
-	// doesn't depend on the original one. (issue 1345)
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
-	outreq = outreq.WithContext(ctx)
-
 	res, err := transport.RoundTrip(outreq)
 	if err != nil {
 		return err