Added regression test for attribute definition on ruby 1.8.7

There is a bug when using ruby 1.8.7 in [this line](https://github.com/fog/fog-core/blob/master/lib/fog/core/attributes.rb#L102).

In newer versions the code 

```ruby
[*options[:aliases]]
```

return an empty array, but on 1.8.7 it returns an array with nil as an element. The error is reproduced on a build in [fog/fog-xenserver](https://travis-ci.org/fog/fog-xenserver/builds/29160938).

I have already fixed it, but i will send just the regression test first and let the build fail. After that, i will send the code fix. =)