[FIX] base_calendar: correct the invert call to the function field rrule.
Create new hook method with valid signature.
Keep the old one for compatibility reasons.
Force the value of 'byday' (type selection) to be a string instead of integer.
[FIX] if only one id, convert to list
[FIX] pad: use server to get pad content
[IMP] add test
[MERGE] [FIX] base_calendar: correct the set method for rrule (opw 605088)

- create hook method to fix the invalid call while keeping the signature of the old methdo
- remove undeclared variable
- force the 'byday' field (type selection) to be a string value instead of an integer