fix incorrect specs for attribute merging

in chef, attributes defined at the same precedence levels get deep-merged. attributes
defined at a higher precedence level clobber the attributes at a lower precedence level.
here's an example from a `chef-shell`:

```
chef > node.default.igor = {"a" => "mike", "b" => "joe", "c" => [1, 2, 3]}
 => {"a"=>"mike", "b"=>"joe", "c"=>[1, 2, 3]}
chef > node.override.igor = {"b" => "bob", "c" => [99, 98, 97]}
 => {"b"=>"bob", "c"=>[99, 98, 97]}
chef > node.igor
 => {"a"=>"mike", "b"=>"bob", "c"=>[99, 98, 97]}
```

you can see that the value for `node.igor.c` is NOT `[1,2,3,99,98,97]` but instead just
`[99, 98, 97]`, the value defined at the override level.

the specs here incorrectly stipulate that security groups defined at an override level
should be merged with those specified at the default level, which is incorrect.
fix failing spec for attribute merging
Merge pull request #49 from airbnb/proper_attribute_merging

Proper attribute merging