decodeMapFromSlice should ignore ZeroFields when not first element

[timestee]

Thanks for this project, I've had a config file merge and load implementation using this project, and got a wired problem: sandwich-go/xconf#6

mapstructure/mapstructure.go

Lines 823 to 830 in b9b99d7

	for i := 0; i < dataVal.Len(); i++ {
	err := d.decode(
	name+"["+strconv.Itoa(i)+"]",
	dataVal.Index(i).Interface(), val)
	if err != nil {
	return err
	}
	}

Should decodeMapFromSlice ignore ZeroFields when i != 0 ? Otherwise, the merged map will be clean again.