It wasn't lowercasing the string, folding widths, and putting strings
into NFC form. Do those.

Fixes #13835

Change-Id: Ia3de6159417cacec203b48e206e51d79f945df58
Reviewed-on: https://go-review.googlesource.com/29860
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>