BYDAY iCal property should be in the timezone of the event

[stephenharris]

Originally reported here: https://wordpress.org/support/topic/ical-weekly-recurrence-and-timezone-issue?replies=2

Prior to 3.0.0 and #293, all date-times in the generated iCal feed were converted to UTC timezone. Corresponding the day of an event may change (e.g. UTC-4 event at 10pm on a Monday is a Tuesday event in UTC). (The BYDAY property should reflect the day in the timezone of the event.)

However, with #293, if the user is using a recognisable timezone (not UTC offset) we keep the event in that timezone, so we should not change the BYDAY property in that context.