fix youtube id filtering

If the youtube link is based on short version like `http://youtu.be/UowkIRSDHfs` the ids were not filtered out properly. This will fix this issue.

lib/youtube-dl.js

@@ -85,7 +85,7 @@ function call(video, args1, args2, options, callback) {
  } else {
  // Get possible IDs for youtu.be from urladdr.
  id = details.pathname.slice(1).replace(/^v\//, '');
- if (id === 'playlist') {
+ if (id || id === 'playlist') {
  args.push(video);
  }
  }