Don't panic when a http.ResponseWriter does not implement CloseNotifier

Instead, provide a variant of instrumentedResponseWriter that does not
implement CloseNotifier, and use that when necessary. In
copyFullPayload, log instead of panicing when we encounter something
that doesn't implement CloseNotifier.

This is more complicated than I'd like, but it's necessary because
instrumentedResponseWriter must not embed CloseNotifier unless there's
really a CloseNotifier to embed.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

registry/handlers/helpers.go

@@ -29,7 +29,7 @@ func copyFullPayload(responseWriter http.ResponseWriter, r *http.Request, destWr
 	if notifier, ok := responseWriter.(http.CloseNotifier); ok {
 		clientClosed = notifier.CloseNotify()
 	} else {
-		panic("the ResponseWriter does not implement CloseNotifier")
+		ctxu.GetLogger(context).Warn("the ResponseWriter does not implement CloseNotifier")
 	}
 
 	// Read in the data, if any.