EZP-20979: enable Legacy Storage to implicitly create fields for main and initial language
EZP-20979: fixed: sortKeyInt is not nullable
EZP-20979: fixed: restore sort key for default value
Added comment and todo
EZP-20979: fixed field type not registered
EZP-20979: implement tests for passing non-redunant set of fields to storage
EZP-20979: update phpdoc