add proper conversion for *struct

Hi, I find bug for converting *Struct.

Here is my test code.

```go
package main

import "fmt"
import "github.com/stevedonovan/luar"

const test = `
	Print(NewTest(0) == nil) -- expected true, but false
`

type Test struct {
}

func NewTest(i int) *Test {
	if i == 0 {
		return nil
	}
	return &Test{}
}

func main() {
	L := luar.Init()
	defer L.Close()

	luar.Register(L, "", luar.Map{
		"Print": fmt.Println,
		"NewTest":   NewTest,
	})

	L.DoString(test)
}

```