bug#39782: 28.0.50; gnus-icalendar does not understand multiple repeating days

[Mark A. Hershberger]

I used gnus-icalendar's "Export to Org" feature on an appointment, and
it successfully made a repeating entry (GOOD!) but failed to catch the
days other than the first one (BAD!).

The created org-entry contains the following RRULE entry:

    :RRULE:    
FREQ=WEEKLY;UNTIL=20200519T133000Z;INTERVAL=1;BYDAY=MO,TU,WE,TH,FR;WKST=SU

The only entry created, though, was

    <2020-02-25 09:30-10:00 +1w>

I had to manually add the following entries:

  <2020-02-26 09:30-10:00 +1w>
  <2020-02-27 09:30-10:00 +1w>
  <2020-02-28 09:30-10:00 +1w>
  <2020-03-02 09:30-10:00 +1w>

>From skimming gnus-icalendar.el and icalendar.el, it looks like the
"BYDAY" parsing in icalendar.el only looks for one day, instead of the
multiples given.  I think gnus-icalendar.el's export to org functions
are only set up to insert one day instead of the multiple times required
by this rule.