The jarKey function handles broken PublicSuffixList implementations but
no test verified it.

Change-Id: Ifb76de9e8c3941f3b08d3e43970056e023013457
Reviewed-on: https://go-review.googlesource.com/38357Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>