Add "array" type hints

This creates a regression because of the requirement for signatures to match in object inheritance hierarchies. There might be other issues, but the one I'm getting is:
Warning: Declaration of webform_exporter_excel_xlsx::add_row(&$file_handle, $data, $row_count) should be compatible with webform_exporter::add_row(&$file_handle, array $data, $row_count) in include_once() (line 6 of /var/www/drupal7/sites/all/modules/contrib/webform/includes/exporters/webform_exporter_excel_xlsx.inc).

Committed to 7.x-4.x

@Liam Morland: Each maintainer needs to arrive at a maintenance philosophy that he/she is comfortable with. I tended to *not* make code changes for "clean up" purposes without significant community review. There isn't much upside and there is some downside, as this issue demonstrates. The choice is yours to make, of course. I'm just back-seat driving. :) Many thanks for all your work on webform.