occurs_between? misses start time when duration is zero

[avit]

The comment says "inclusive" but it actually excludes the start time when duration is zero:

schedule = IceCube::Schedule.new(Time.utc(2013, 1, 1))
t1, t2 = Time.utc(2013, 2, 10), Time.utc(2013, 2, 11)
schedule.add_recurrence_time t1

schedule.occurring_between?(t1, t2) # => false
schedule.duration = 1
schedule.occurring_between?(t1, t2) # => true