API cart attributes

// ---------------------------------------------------------
// POST to cart/update.js returns the cart in JSON.
// To clear a particular attribute, set its value to an empty string.
// Receives attributes as a hash or array. Look at comments below.
// ---------------------------------------------------------
Shopify.updateCartAttributes = function(attributes, callback) {
  var data = '';
  // If attributes is an array of the form:
  // [ { key: 'my key', value: 'my value' }, ... ]
  if (jQuery.isArray(attributes)) {
    jQuery.each(attributes, function(indexInArray, valueOfElement) {
      var key = attributeToString(valueOfElement.key);
      if (key !== '') {
        data += 'attributes[' + key + ']=' + attributeToString(valueOfElement.value) + '&';
      }
    });
  }
  // If attributes is a hash of the form:
  // { 'my key' : 'my value', ... }
  else if ((typeof attributes === 'object') && attributes !== null) {
    jQuery.each(attributes, function(key, value) {
        data += 'attributes[' + attributeToString(key) + ']=' + attributeToString(value) + '&';
    });
  }
  var params = {
    type: 'POST',
    url: '/cart/update.js',
    data: data,
    dataType: 'json',
    success: function(cart) {
      if ((typeof callback) === 'function') {
        callback(cart);
      }
      else {
        Shopify.onCartUpdate(cart);
      }
    },
    error: function(XMLHttpRequest, textStatus) {
      Shopify.onError(XMLHttpRequest, textStatus);
    }
  };
  jQuery.ajax(params);
};