message: Handle setting a newTag without metaData
  metaData is actually the request body for the PUT that adds the tag
  so we need to send {} for e.g. m.lowpriority, which is not manually
  ordered.
diff: @@ -114,7 +114,7 @@ RoomListActions.tagRoom = function(matrixClient, room, oldTag, newTag, oldIndex,
  (hasChangedSubLists || metaData)
  ) {
  // Optimistic update of what will happen to the room tags
  - room.tags[newTag] = metaData;
  + room.tags[newTag] = metaData || {};
  const promiseToAdd = matrixClient.setRoomTag(roomId, newTag, metaData).catch(function(err) {
  const ErrorDialog = sdk.getComponent("dialogs.ErrorDialog");
  