code: func NewMongoStore(c *mgo.Collection, maxAge int, ensureTTL bool,
  	keyPairs ...[]byte) *MongoStore {
  	store := &MongoStore{
  		Codecs: securecookie.CodecsFromPairs(keyPairs...),
  		Options: &sessions.Options{
  			Path:   "/",
  			MaxAge: maxAge,
  		},
  		Token: &CookieToken{},
  		coll:  c,
  	}
  
  	store.MaxAge(maxAge)
  
  	if ensureTTL {
  		c.EnsureIndex(mgo.Index{
  			Key:         []string{"modified"},
  			Background:  true,
  			Sparse:      true,
  			ExpireAfter: time.Duration(maxAge) * time.Second,
  		})
  	}
  
  	return store
  }
docs: // NewMongoStore returns a new MongoStore.
  // Set ensureTTL to true let the database auto-remove expired object by maxAge.