dev/core#651 Fix group by on export soft credits (possible recent regression, clearly...

dev/core#651 Fix group by on export soft credits (possible recent regression, clearly wrong).

As pointed out by the reporter the group by is being calculated as if it were a string but it's an array, this fixes.

This code has been touched recently so it might be a recent regression. 5.10 is the first release in a long time where
export is working in some mysql / output configs after a big refactor to get rid of wide temp tables