enable mass assignment protection in test app; fix failed tests caused b...

...y mass-assignment exception

There are still two failed tests. One of which is as follow. not sure how to fix it.

```
test_should_allow_admin_to_sticky_and_lock(TopicsControllerTest):
ActiveModel::MassAssignmentSecurity::Error: Can't mass-assign protected attributes: body
    /Users/joshua/jsh-c-github/communityengine/app/controllers/topics_controller.rb:51:in `create'
```
doesnt look like the following config is sufficient?
topic.rb
```ruby 
attr_accessible :sb_posts_attributes
```
sb_post.rb 
```ruby 
attr_accessible :body
```