Bug: op_UnaryPlus on int16 overlooked

```
Found bug:

<@ (~+) 3s @> |> eval;;

fails. I am currently refactoring native numeric and conversion op 
implementations so that there is more code reuse and less change for these 
kinds of typo / oversight bugs.


```

Original issue reported on code.google.com by `stephen....@gmail.com` on 10 Jul 2011 at 9:20
