Fix :args instead of :array when there are args and assocs

Turns out #125 is actually just an issue with array literals in general:

```
1.9.3-p392 :002 > Ruby20Parser.new.parse "[1, 2 => 3]"
 => s(:args, s(:lit, 1), s(:hash, s(:lit, 2), s(:lit, 3))) 
```