This allows slices of custom types with byte as underlying type to be
decoded, fixing a regression introduced in CL 9371.

Fixes #12921.

Change-Id: I62a715eaeaaa912b6bc599e94f9981a9ba5cb242
Reviewed-on: https://go-review.googlesource.com/16303Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>