False-valued reductions/extracts are not rendered in CSV exports.

[hughdickinson]

The check against present on this line:

caesar/app/models/exporters/csv_exporter.rb

Line 58 in 4c0ca58

return "" unless item.present?

returns false for false valued boolean data.

This means that reducers which return boolean valued data e.g.

{"result": false}

Will produce an empty column in the resultant data export for all reductions that return false.

The expected result is that a literal false value should be output.