More Rubocop warnings fixed

It is now following `Fog` main repo style guide.


I changed this because of those rubocop warnings:

```ruby
lib/fog/core/attributes.rb:56:31: C: Pass &:to_s as an argument to collect instead of a block.
        @ignored_attributes = args.collect {|attr| attr.to_s }
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
lib/fog/core/attributes.rb:56:36: C: Prefer map over collect.
        @ignored_attributes = args.collect {|attr| attr.to_s }
                                   ^^^^^^^
```

I do prefer the `&:` over the `{ |attr| }`, so i guess it is a matter of opinion.
What if we keep rubocop style for now, and rethink this in the future (another pull request)?