Recurrence expansion does not include DTSTART when not covered by the RRULE

I ran into this issue because the calendar app on newer iPads, probably mistakingly, allows the creation of an event where the start date is not covered by the `RRULE`. It's possible to create a recurring event that eg. starts on a Wednesday, but has a weekly recurrence rule of only Thursdays.

Apple's iCalendar application and Google Calendar both display that oddball first instance, but my application (which uses ical.js) does not.

[The RFC5545 section about RRULE](https://tools.ietf.org/html/rfc5545#section-3.3.10) doesn't engrave in stone what's supposed to happen, but it does say (as part of the spec for `COUNT` in an `RRULE`):

> The "DTSTART" property value always counts as the first occurrence.

... So I'm fairly certain that this is a bug in ical.js.

Here's a reduced test case:

```js
const icalJs = require('ical.js');
const vevent = new icalJs.Event(new icalJs.Component(icalJs.parse(`
BEGIN:VCALENDAR
CALSCALE:GREGORIAN
METHOD:REQUEST
VERSION:2.0
BEGIN:VEVENT
CLASS:PUBLIC
CREATED:20160920T065356Z
DTSTART:20170301T100000
DTEND:20170301T110000
RRULE:FREQ=WEEKLY;BYDAY=TH
SUMMARY:Happens every Thursday, but starts on a Wednesday
UID:C2366E0E-09A4-4B9C-B9F5-EE96CEE467F0
END:VEVENT
END:VCALENDAR
`)).getFirstSubcomponent('vevent'));
const iterator = vevent.iterator();

let next;
let num = 0;
while ((next = iterator.next()) && num++ < 5) {
console.log(vevent.getOccurrenceDetails(next).startDate.toICALString());
}
```

Output:

```
20170302T100000
20170309T100000
20170316T100000
20170323T100000
20170330T100000
```

Expected output:

```
20170301T100000
20170302T100000
20170309T100000
20170316T100000
20170323T100000
```

Reproduce the issue with the provided ical.js snippet, starting at vevent.iterator() and getOccurrenceDetails(). Trace how the recurrence iterator handles DTSTART when RRULE does not include it, then add a regression test that expects the DTSTART occurrence before the RRULE dates.

Written by the indexing model from the issue text.