Bug 1793757 - CalICSProvider should request ics data, not xml. r=lasana

When requesting text/xml, servers would and should should return HTTP 406.

Differential Revision: https://phabricator.services.mozilla.com/D158821

--HG--
extra : rebase_source : d9f54e266d737331781480474edd99043e570ac1
extra : amend_source : aaa9a1e2850ca26a7fcff355d0fe9c10504888d0

calendar/providers/caldav/modules/CalDavRequest.jsm

@@ -231,6 +231,7 @@ class CalDavRequestBase {
     copyHeader("Recipient");
     copyHeader("If-None-Match");
     copyHeader("If-Match");
+    copyHeader("Accept");
 
     aNewChannel.requestMethod = oldHttpChannel.requestMethod;
     this.session.prepareRedirect(aOldChannel, aNewChannel).then(() => {

calendar/providers/ics/CalICSProvider.jsm

@@ -274,7 +274,9 @@ class ICSDetector {
    * @return {Promise<calICalendar[] | null>}   An array of calendars or null.
    */
   async _attemptMethod(method, location) {
-    let request = new CalDavGenericRequest(this.session, null, method, location);
+    let request = new CalDavGenericRequest(this.session, null, method, location, {
+      Accept: "text/calendar, application/ics, text/plain;q=0.9",
+    });
 
     // `request.commit()` can throw; errors should be caught by calling functions.
     let response = await request.commit();