Test splat assignment from a subclass of Array to a single lvalue

This is designed to catch a couple differences in behavior I have observed between MRI and Rubinius. In MRI irb:

```
2.1.1 :001 > class ArraySubclass < Array; end
 => nil 
2.1.1 :002 > a = *(ArraySubclass.new([1,2]))
 => [1, 2] 
2.1.1 :003 > a.class
 => Array
```

In rbx:

```
rbx-2.2.9 :001 > class ArraySubclass < Array; end
 => nil 
rbx-2.2.9 :002 > a = *(ArraySubclass.new([1,2]))
 => [1, 2] 
rbx-2.2.9 :003 > a.class
 => ArraySubclass 
```

In short, when doing a splat assignment to a single lvalue, Rubinius just assigns a reference to the *splatted array*. MRI *always* creates a new Array.

(I am working on a gem where this minor difference causes the tests to fail on rbx.)