title: Spring Boot MongoDB Indexed with expireAfterSeconds to Auto delete Document does not work
body: <p>I have a problem with the "time to live" settings in MongoDB. I created an Indexed Annotation in my Spring-Boot 2.0.2.RELEASE project in my Entity which represents my Document in the MongoDB. I set the "expireAfterSeconds" for testing to 15 seconds but MongoDB does not delete a inserted Document after 15 seconds. Can someone tell me what I'm doing wrong?</p>
  
  <p>This is the MongoDB Index as JSON:</p>
  
  <pre><code>[
    2,
    {
      "createdDateTime" : 1
    },
    "deleteAt",
    "AccountServiceDB.AccountRegistration",
    NumberLong(15)
  ]
  </code></pre>
  
  <p>This is my entity:</p>
  
  <pre><code>@Document(collection = "AccountRegistration")
  public class UserRegistration {
  
    @Id
    private ObjectId _id;
    @Indexed(unique = true)
    private String username;
  
    @Indexed(unique = true)
    private String email;
  
    private String user_password;
  
    @Indexed(name = "deleteAt", expireAfterSeconds = 15)
    private Date createdDateTime;
  
    public UserRegistration() {}
  
    public ObjectId get_id() {
      return _id;
    }
  
    public void set_id(ObjectId _id) {
      this._id = _id;
    }
  }
  </code></pre>