Add support writing the header if a slice is empty

If slice is empty, the header is not output. Are there plans to support outputting it?

Sample code:
```go
func main() {
	users := []struct {
		Name string
		Age  int
	}{}

	var buf bytes.Buffer
	w := csv.NewWriter(&buf)
	csvutil.NewEncoder(w).Encode(users)
	w.Flush()

	fmt.Println(buf.String())
}
```

Want:
```
Name,Age
```

Actual:
```

```