The "string" option only applies for strings, floats, integers, and
booleans as per the documentation. So when decoding ignore the "string"
option if the value is not of one of the types mentioned. This matches
the Marshal step which also ignores the "string" option for invalid
types.

Fixes #9812

Change-Id: I0fb2b43d0668bc0e2985886d989abbf2252070e2
Reviewed-on: https://go-review.googlesource.com/10183Reviewed-by: Russ Cox <rsc@golang.org>