In other systems, putting a leaf certificate in the root store works to
express that exactly that certificate is acceptable. This change makes
that work in Go too.

Fixes #16763.

Change-Id: I5c0a8dbc47aa631b23dd49061fb217ed8b0c719c
Reviewed-on: https://go-review.googlesource.com/27393
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>