We only support unmarshaling into a string or a []byte, but we
previously would try (and panic while) setting a slice of a different
type. The docs say ",innerxml" is ignored if the type is not string or
[]byte, so do that for other slices as well.

Fixes #15600.

Change-Id: Ia64815945a14c3d04a0a45ccf413e38b58a69416
Reviewed-on: https://go-review.googlesource.com/32919
Run-TryBot: Quentin Smith <quentin@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>