Change CSVBuilder to get column name from I18n

* Will try to get the column through Model.human_attribute_name.
* If the attribute name is not defined on the locale, the returned name will no longer be "titleized", it will be "humanized".
* If the value specified to the `column` method is not a symbol, it won't get the column name through I18n.
* Very useful when using locales. :)
Rename csv builder specs for better understanding
Merge pull request #2771 from mateusg/csv_column_improvement

Change CSVBuilder to get column name from I18n