allow for an optional end filtering date for comment stats

Is it possible to "splat" these arguments? Something like:
```
filter_args = {
    :course_id => course_id,
    :anonymous => false,
    :anonymous_to_peers => false
}
if end_date then
   filter_args.update({:created_at.lte => end_date})

if user_id != "*" then
   filter_args.update({:author_id => user_id})

content = Content.where(*filter_args)
```

Not sure if that's possible - memory does not serve well and is not satisfied by quick search... either way, what you have here works just fine, just might be a cleaner read if arguments are splatted