Functions like ToLower and ToUpper return the invalid rune back,
so we might as well do the same here.

I changed my mind about panicking when I tried to document the behavior.

Fixes #16690 (again).

Change-Id: If1c68bfcd66daea160fd19948e7672b0e1add106
Reviewed-on: https://go-review.googlesource.com/30935
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>