Use correct values for attributes_to_show

When fetching COUNTER R5 IR reports with some usage data providers get the following error:

Report not valid: [{"Code":3062,"Severity":"Warning","Message":"Invalid ReportAttribute Value","Data":"Attribute parameter: Attributes_To_Show has invalid value(s): [Author]."}

According to the SUSHI Swagger docs the parameter should be named `Authors` rather than `Author`.

This relates to #MODEUSHARV-35