caldav: return SupportedComponentSet in PROPFIND

I started using this project to export tasks over CalDav, more
specifically to Reminders on iOS/macOS. I quickly realized that
even if you specify that `SupportedComponentSet` contains `VTODO`, that
isn't reflected properly when doing the `PROPFIND`.

This patch should fix that, while keeping the behaviour of defaulting to
`VEVENT` for propfind. Also added some tests to make sure that I didn't
break anything (Which I hope I didn't 😅).

caldav/server.go

@@ -525,10 +525,16 @@ func (b *backend) propFindCalendar(ctx context.Context, propfind *internal.PropF
 			}, nil
 		},
 		supportedCalendarComponentSetName: func(*internal.RawXMLValue) (interface{}, error) {
+			components := []comp{}
+			if cal.SupportedComponentSet != nil {
+				for _, name := range cal.SupportedComponentSet {
+					components = append(components, comp{Name: name})
+				}
+			} else {
+				components = append(components, comp{Name: ical.CompEvent})
+			}
 			return &supportedCalendarComponentSet{
-				Comp: []comp{
-					{Name: ical.CompEvent},
-				},
+				Comp: components,
 			}, nil
 		},
 	}