Default handler for setRoomJoinRule

May resolve issue #247, needs further testing.

src/components/roomInfoMixin.js

@@ -79,6 +79,9 @@ export default {
     },
   },
   methods: {
+    setRoomJoinRule(ignoredJoinRule) {
+      // Do nothing in the general mixin, override in RoomInfo.vue
+    },
     getRoomJoinRule() {
       return this.$matrix.getRoomJoinRule(this.room);
     },