Fixed Opml::Outline#flatten

before:

```
/Users/ykzts/.rvm/rubies/ruby-1.9.3-p385/bin/ruby -S rspec ./spec/opml_spec.rb
...............................F

Failures:

  1) Opml::Outline in playlist should return flattened child outlines
     Failure/Error: ]
       expected: ["The Last Napster Sunday?", "Heart of Glass.mp3", "Manic Monday.mp3", "Everybody Have Fun Tonight.mp3", "She Blinded Me With Science.mp3", "Rivers of Babylon   (HTC).mp3", "The Tide Is High.mp3", "Back to the Island.mp3", "Lucky Man.mp3", "Up on Cripple Creek.mp3", "Crackerbox Palace.mp3", "Taxi.Mp3", "Thick As A Brick.mp3", "Riding With the King.mp3"]
            got: ["The Last Napster Sunday?", "[Heart of Glass.mp3]", "[Manic Monday.mp3]", "[Everybody Have Fun Tonight.mp3]", "[She Blinded Me With Science.mp3]", "[Rivers of Babylon   (HTC).mp3]", "[The Tide Is High.mp3]", "[Back to the Island.mp3]", "[Lucky Man.mp3]", "[Up on Cripple Creek.mp3]", "[Crackerbox Palace.mp3]", "[Taxi.Mp3]", "[Thick As A Brick.mp3]", "[Riding With the King.mp3]"] (using ==)
     # ./spec/opml_spec.rb:200:in `block (2 levels) in <top (required)>'

Finished in 0.36698 seconds
32 examples, 1 failure

Failed examples:

rspec ./spec/opml_spec.rb:184 # Opml::Outline in playlist should return flattened child outlines
```