encoding/json: don't marshal unexported embedded fields of non struct type
Marshal must process unexported embedded fields of struct type,
looking for exported fields in those structs. However, it must
not process unexported embedded fields of non-struct type.
For example, consider:
type t1 struct {
X int
}
type t2 int
type T struct {
t1
t2
}
When considering T, Marshal must process t1 to find t1.X.
Marshal must not process t2, but it was. Fix that.