proxy: recognize client's cancellation

fix issue #1589

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

caddyhttp/httpserver/error.go

@@ -8,6 +8,7 @@ var (
 	_ error = NonHijackerError{}
 	_ error = NonFlusherError{}
 	_ error = NonCloseNotifierError{}
+	_ error = NonPusherError{}
 )
 
 // NonHijackerError is more descriptive error caused by a non hijacker
@@ -42,3 +43,14 @@ type NonCloseNotifierError struct {
 func (c NonCloseNotifierError) Error() string {
 	return fmt.Sprintf("%T is not a closeNotifier", c.Underlying)
 }
+
+// NonPusherError is more descriptive error caused by a non pusher
+type NonPusherError struct {
+	// underlying type which doesn't implement pusher
+	Underlying interface{}
+}
+
+// Implement Error
+func (c NonPusherError) Error() string {
+	return fmt.Sprintf("%T is not a pusher", c.Underlying)
+}