Fixed array.unshift

Another fix. 

Before:

```
$ shine
Shine 0.0.2 -- Copyright (C) 2013-2014 Richard Hundt.
shine> a = [1, 2, 3]
shine> a.unshift(4)
[string "core"]:0: attempt to perform arithmetic on global 'l' (a nil value)
```

After:

```
$ shine
Shine 0.0.2 -- Copyright (C) 2013-2014 Richard Hundt.
shine> a = [1, 2, 3]
shine> a.unshift(4)
shine> print(a)
[4,1,2,3]
```