Corrects rendering of monthly frequencies on weekdays

Rules with monthly frequencies that only occur Monday to Friday renders without the 'on weekdays' part. For example: ```FREQ=MONTHLY;DTSTART=20140119T090000Z;INTERVAL=1;WKST=1;BYDAY=MO,TU,WE,TH,FR``` shows up as ```every month``` instead of  ```every month on weekdays```.

This patch fixes that.