WebStat: fix for several minor issues

• Export to Excel: in order to be able to export non-ascii characters the workbook needs to be created with encoding 'utf-8'.

• Update the code that was assuming that all the recids that are in the circulation tables are also in bibrec table (which is not entirely true, ex. recid=0).

• Update the code that was assuming that the 'collection' value is always present in the args (which is not always true).