title: Google does'nt sync my subscribed ICS feed
body: <p>I have a ICS feed for a calendar, that i subscribe to from Google calendar.
  As soon as i add the feed, it shows all the events perfectly.</p>
  
  <p>But when i edit events, it does'nt update. 
  So it seems like it can read the feed, but does'nt sync it.</p>
  
  <p>If i remove the feed and add it again, its still not synced.
  But if i add a paramater to the url (ex. &amp;x=1), it will import the updates, but still not sync.
  So it seems like Google is caching the feed pr. url, but does'nt sync after the url has been added.</p>
  
  <p>Anyone else had this issue, or know a solution?</p>
  
  <p><strong>SOLUTION UPDATE: 12/03 - 2020</strong></p>
  
  <p>I missed the fields "SEQUENCE" and "LAST-MODIFIED". Credits to @anmari for spotting the error.</p>
  
  <p>The calendar now syncs perfectly with Google.</p>
  
  <p>Example from the ICS file:</p>
  
  <pre><code>BEGIN:VCALENDAR
  VERSION:2.0
  PRODID:-//ZContent.net//Zap Calendar 1.0//EN
  CALSCALE:GREGORIAN
  METHOD:PUBLISH
  BEGIN:VEVENT
  SUMMARY:Gudstjeneste - prædiken ved Enok Sørensen
  UID:1177
  DTSTART:20190915T073000Z
  DTEND:20190915T103000Z
  DTSTAMP:20190907T164050Z
  LAST-MODIFIED:20200311T195313Z
  SEQUENCE:2
  LOCATION:Nørregade 13, 6900 Skjern
  URL:https://www.lm-kirken.dk/kalender/gudstjeneste-15-09/
  END:VEVENT
  BEGIN:VEVENT
  SUMMARY:Fredagsfællesskab
  UID:1248
  DTSTART:20190920T153000Z
  DTEND:20190920T180000Z
  DTSTAMP:20190909T201436Z
  LAST-MODIFIED:20200312T062507Z
  SEQUENCE:1
  LOCATION:Nørregade 13, 6900 Skjern
  URL:https://www.lm-kirken.dk/kalender/fredagsfaellesskab-20-09/
  END:VEVENT
  BEGIN:VEVENT
  SUMMARY:Gudstjeneste - prædiken ved Leif Bach Kofoed
  UID:1214
  DTSTART:20190929T083000Z
  DTEND:20190929T103000Z
  DTSTAMP:20190909T185328Z
  LAST-MODIFIED:20200311T210357Z
  SEQUENCE:1
  LOCATION:Nørregade 13, 6900 Skjern
  URL:https://www.lm-kirken.dk/kalender/gudstjeneste-29-9/
  END:VEVENT
  END:VCALENDAR
  </code></pre>