And adds missing tests.

Fixes #14970

Change-Id: I0dba02603bc245f555498cb5dd3e0a9d87c52353
Reviewed-on: https://go-review.googlesource.com/21467Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>