Patch originally from Steven Hartland. Tweaked a bit & added a test.

Fixes #7197

Change-Id: I09012b4674e7c641dba31a24e9758cedb898d3ee
Reviewed-on: https://go-review.googlesource.com/21196Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>