Return / for principal-collection-set

From RFC3744: This protected property of a resource contains a set of URLs that identify the root collections that contain the principals that are available on the server that implements this resource.

radicale/xmlutils.py

@@ -592,8 +592,11 @@ def _propfind_response(path, item, props, user, write=False):
             element.text = item.etag
         elif tag == _tag("D", "getlastmodified"):
             element.text = item.last_modified
-        elif tag in (_tag("D", "principal-collection-set"),
-                     _tag("C", "calendar-user-address-set"),
+        elif tag == _tag("D", "principal-collection-set"):
+            tag = ET.Element(_tag("D", "href"))
+            tag.text = _href(collection, "/")
+            element.append(tag)
+        elif tag in (_tag("C", "calendar-user-address-set"),
                      _tag("D", "principal-URL"),
                      _tag("CR", "addressbook-home-set"),
                      _tag("C", "calendar-home-set")):