Bugfix - Apply dtstart option when forceset: true

[jutaz]

This PR fixes a bug which happens when using rrulestr in a similar way to this:

rrulestr('RRULE:FREQ=WEEKLY', {
  forceset: true,
  dtstart: new Date()
});

In the past (talking ~2.2.0 ish) this would apply dtstart and the created rules would have the dtstart. That's no longer the behavior. Having an explicit dtstart which could even override the DTSTART: specified in the rule was quite convenient, and this PR brings it back.

What's interesting is that this is already the behavior when one doesn't specify forceset: true.

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[jbuck94]

@jakubroztocil please merge this PR. This bug is blocking my ability to use the library