secure/precis: use toLower instead of case folding

Change default case folding to the Unicode toLower operation as
specified in draft-ietf-precis-7700bis-03 and
draft-ietf-precis-7613bis-03.

Also handle Greek final sigma correctly

Change-Id: I8e9fec1fc51d38a1950f95be72e07fb80d9949e4
Reviewed-on: https://go-review.googlesource.com/30254
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>