Also add relevant test.

Fixes #13343.

Change-Id: Ib1e65af1d643d501de89adee3618eddbf6c69c9e
Reviewed-on: https://go-review.googlesource.com/18159Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>