[FIX] the ; in content-disposition is a separator not a terminator

According to RFC 6266,

content-disposition = "Content-Disposition" ":"
                      disposition-type *( ";" disposition-parm )

disposition-parm can't be an empty string, and thus a ; in terminal
position is not legal, even though browsers apparently work around it.

addons/web/controllers/pivot.py

@@ -81,7 +81,7 @@ class TableExporter(http.Controller):
 
         response = request.make_response(None,
             headers=[('Content-Type', 'application/vnd.ms-excel'),
-                    ('Content-Disposition', 'attachment; filename=table.xls;')],
+                    ('Content-Disposition', 'attachment; filename=table.xls')],
             cookies={'fileToken': token})
         workbook.save(response.stream)