title: Can't make mongoose expires/ttl to work
body: <pre><code>var mongoose = require('mongoose'), Cache, cache;
  
  mongoose.connect('mongodb://localhost:27017/test');
  
  Cache = mongoose.model('Cache', mongoose.Schema({
    value: {},
    createdAt: {type: Date, expires: 3600}
  }));
  
  cache = new Cache({
    createdAt: new Date(),
    value: {foo: 'bar'}
  });
  cache.save(function(err, obj) {
    console.log(err, obj);
    process.exit();
  });
  </code></pre>
  
  <p>I'm trying to make the cache get removed after certain time. I waited for more than 3 minutes and the document I inserted did not get deleted at all. Have I missed something?</p>