Fix callbacks

As partially discussed at #148, curb using different callback for each 30x, 40x, and 50x response code.
Handle them if it's necessary, or fallback to `on_complete`. Also refactor feed_spec.rb to reflect this changes.

Now fetch_and_parse should returns Feed object if it's success or return response code when it's failing.
And Feed.update will return updated Feed object if it's success otherwise it will return the same Feed object.

Also make `feed.updated?` consistently returns boolean.