muc: Make `defaults` a function

otherwise also the prototype's defaults gets overwritten.

src/converse-muc.js

@@ -365,7 +365,9 @@
 
             _converse.ChatRoom = _converse.ChatBox.extend({
 
-                defaults: _.extend(_converse.ChatBox.prototype.defaults, {
+                defaults: function () {
+                    return _.extend(_converse.ChatBox.prototype.defaults, {
+                        'type': CHATROOMS_TYPE,
                         // For group chats, we distinguish between generally unread
                         // messages and those ones that specifically mention the
                         // user.
@@ -374,10 +376,9 @@
                         // _converse.ChatBox to indicate unread messages which
                         // mention the user and `num_unread_general` to indicate
                         // generally unread messages (which *includes* mentions!).
-                    // FIXME: figure out why this breaks tests.
-                    // 'type': CHATROOMS_TYPE,
                         'num_unread_general': 0
-                }),
+                    });
+                },
 
                 isUserMentioned: function (message) {
                     /* Returns a boolean to indicate whether the current user