And add a test.

Updates #12580

Change-Id: Ia7eaba09b8e7fd0eddbcaefb948d01ab10af876e
Reviewed-on: https://go-review.googlesource.com/22659Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>