encoding/japanese: make iso2022jp replace with FFFD on error This CL also fixes a bug in the JIS 212 mode. Updates golang/go#18898 Change-Id: Idb10e375bbc5d278db4e8550f26986a5d2bb6caa Reviewed-on: https://go-review.googlesource.com/37595 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Nigel Tao <nigeltao@golang.org>