fix double WEEKLY string in to_ical result

Hello :)

I was trying to generate iCal format from schedule which I earlier loaded from yaml. I stumbled across ```FREQ=WEEKLY,WEEKLY``` results from to_ical. They are incompatible with google calendar to which I tried to import my calendar, and probably with RFC (I didn't check that).
After a bit of digging I found that WeeklyRule has doubled ```interval``` values which happens while loading yaml. I wrote a test and fixed it, let me know if it needs refactoring :)