SQL syntax error in GROUP BY clause: CRM_Report_Form::groupBy sets array instead of string

The current groupBy() method sets $_groupBy as an array of group-by definitions, but the buildQuery() method is expecting a fully built group by clause starting with "GROUP BY". This results in queries like "SELECT * FROM civicrm_foo WHERE (1) Array" (instead of "SELECT * FROM civicrm_foo foo WHERE (1) GROUP BY foo.id") so of course we get an SQL syntax error.

The current code appears to have been added for CRM-8503, before which this method was just a placeholder that needed to be overridden by the extending report class – I'm guessing this is why the bug has not been spotted until now.