Fix Alarm related to DTSTART of VTODO

[tpayen]

Following RFC 5545 (https://www.rfc-editor.org/rfc/rfc5545#page-74) the start of a VTODO is DTSTART not DUE
Fixing the tests in consequence

[jarikmarwede]

I have just noticed the same problem to be the cause of an error in my application and have come to the same conclusion for fixing it.
Is there a reason this isn't merged yet, or can I somehow help in getting this merged? @evert
Sorry, if the mention isn't correct. You are the one marked as the author in the PHPDoc comment for the VAlarm class.

[jarikmarwede]

It also seems to me like this is a duplicate of #370, although that PR isn't merged either because it was missing tests in 2017 and now has merge conflicts.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.78%. Comparing base (e1cc246) to head (d734e21).
Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #616      +/-   ##
============================================
- Coverage     98.78%   98.78%   -0.01%     
+ Complexity     1875     1874       -1     
============================================
  Files            71       71              
  Lines          5273     5270       -3     
============================================
- Hits           5209     5206       -3     
  Misses           64       64              

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[phil-davis]

Note for myself and other readers.
I wondered about the code in the "else" that deals with "not START". That has a similar if-else that chooses property "DUE" or "DTEND". The RFC (see link above) has this text in it:

"In an alarm in a "VTODO" calendar component set
to trigger on the "END" of the to-do, either the "DUE" property
MUST be present, or the "DTSTART" and "DURATION" properties MUST
both be present."

So the RFC does expect that there can/must be a "DUE" property. So that code looks reasonable.

Maybe there should be other code somewhere that handles a "DTSTART" and "DURATION" combination. I haven't looked hard for that. And it is out-of-scope for this fix anyway.