Exception when count option used to enumerate non-recurring event.

This was reported by kjwierenga who provided the fix.

Failing test case (modified from kj's original)

in occurrence_enumerator_spec.rb

describe "for a non-recurring event" do
  before(:each) do
    @event_start = Time.now.utc
    @event = RiCal.Event do |event|
      event.dtstart = @event_start
      event.dtend   = @event_start + 3600
      # event.rrule  (no recurrence, single event)
    end
  end

  #Bug reported by K.J. Wierenga
  it "should not raise a NoMethodError when specifying just the :count option" do
    lambda {
      @event.occurrences(:count => 1)
      }.should_not raise_error
    end
  end
end