code: function update(req, res, next) {
    var body = req.body;
  
    db.tags.update(req.params.tag, body, function(err, tag) {
      if(err) { return next(err); }
  
      res.json(201, tag);
      signals.emit('tag:update', tag);
    });
  }
docs: URL: /tags/:tag
  
  Method: PUT
  
  Events:
  
  * `tag:update` (tag)
  
  Status codes:
  
  * `201` if tag was created successfully
  * `409` if a tag with the same `slug` already exists