Fix marshaler detection

Here's a reproducible case:

```
type T []byte

func (x T) MarshalBencode() ([]byte, error) {
	fmt.Println("THIS GOT CALLED")
	return []byte("x"), nil
}

encoded, err := bencode.EncodeBytes([]interface{}{
	T("something"),
})
fmt.Println(encoded)
```

Notice that MarshalBencode() is not called when it should be.

The problem stems from NumMethod() returning 0 for the interface, even though the `vi.(Marshaler)` cast would succeed. Removing that if check fixed the issue for me.