backbone fetch HEAD request

fetch: function fetch(options) {
  // populate serverTime with the date response header
  options = options || {};
  options.method = 'HEAD';
  options.success = function (model, response, options){
    model.set({
      serverTime: options.xhr.getResponseHeader('Date')
    });
  };
  Backbone.Model.prototype.fetch.call(this, options);
}