Why is rrule lowercase?

[jimryan]

Google Calendar (via iCal) is rejecting ICS files for recurring events generated by this library because the rrule property is lowercase. I'm happy to submit a PR, but I wanted to open an issue and see if it was lowercase for a specific reason. In the ICS files I'm generating, it's the only property that's lowercase.

Failing example:

cal.addEvent('Foo', 'Bar', 'Baz', start, end, {
  freq: 'WEEKLY',
  interval: 4
});

Working example (resolved by writing the property myself with an uppercase RRULE):

cal.addEvent('Foo', 'Bar', 'Baz', start, end, {
  rrule: 'RRULE:FREQ=WEEKLY;INTERVAL=4'
});