toString method
A valid RFC 5545 string representation of this recurrence rule, except
when end is specified as End.Until.
When End.Until is specified, 'UNTIL' holds the total number of microseconds
which need to be added to a desired start date.
'UNTIL' should be reassigned to a calculated end date time, formatted using
the RFC 5545 specifications: https://tools.ietf.org/html/rfc5545#section-3.3.5
Implementation
@override
String toString() {
String rule = 'RRULE:FREQ=${frequency.name}';
rule += (interval != 1) ? ';INTERVAL=$interval' : '';
rule += (end.type != EndType.NEVER) ? ';$end' : '';
return rule;
}