In particular, don't assume that one reflect.Value can be assigned to another just because they have the same reflect.Kind.

Fixes #12401

Change-Id: Ia4605a5c46557ff8f8f1d44f26d492850666c6d1
Reviewed-on: https://go-review.googlesource.com/15420Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>