decode do not zero the field

[cuiweixie]

func TestIssue299(t *testing.T) {
var a []interface{}
inputStringSlice := map[string]interface{}{
"vfoo": "foo",
"vbar": a,
}
var result = Slice{
Vfoo: "a",
Vbar: []string{"a", "b"},
}
config := &DecoderConfig{
Metadata: nil,
ZeroFields: true,
Result: &result,
WeaklyTypedInput: true,
}

decoder, err := NewDecoder(config)
if err != nil {
	t.Fatalf("got error: %s", err)
}

err = decoder.Decode(inputStringSlice)
if err != nil {
	t.Fatalf("got error: %s", err)
}
if result.Vfoo != "foo" {
	t.Errorf("want want get %s", result.Vfoo)
}
if len(result.Vbar) != 0 {
	t.Errorf("want len(result.Vbar) == 0, get %d", len(result.Vbar))
}

}

this will failed