More Rubocop warnings fixed

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

Sure @nirvdrum. Here are the links:

* [Prefer each over for](http://rubydoc.info/gems/rubocop/RuboCop/Cop/Style/For)
* [Favor modifier unless usage when having a single-line body](http://rubydoc.info/gems/rubocop/RuboCop/Cop/Style/IfUnlessModifier)
* [Pass &:to_s as an argument to collect instead of a block](http://rubydoc.info/gems/rubocop/RuboCop/Cop/Style/SymbolProc)
* [Prefer map over collect](http://rubydoc.info/gems/rubocop/RuboCop/Cop/Style/CollectionMethods)